There seem to be divergent opinions on the utility of Harmony (now that it works, v1.07) as well as Fast Block for Artillery and Tank Mages, so I did some calculations that account for differences in style, level, AC and DEX to find some recommendations. The basis and results are shown first, followed by a recommendation list, specific examples, and some of the relevant data. In particular, Warriors and those who have "tried it once and it didn't work for me" should find some interesting results.
Enjoy!

BASIS
For equispaced attacks when surrounded by a mob, the number you can face before a high DEX character gets his autoblock broken is:
n = (Monster swingtime) / (Monster's HitRate vs Character)/(Speed-of-Block)

Formula for stunlock: once block broken through, with how many creatures will stunlock occur?
n = (Monster swingtime) / (Monster's HitRate vs Character) / (Speed-of-Recovery)

Where Speed of block = 0.10 sec for Warriors & Fast Block, 0.20 sec Rogues, 0.30 Sorc.
Speed of Recovery = 0.30 sec (W), 0.35 (R), 0.40 (S), with -0.15 Harmony, -0.10 Stability,
-0.05 Balance, and must add in time for the attempted 'escape', whether a swing with a sword of haste (0.35/0.45/0.50 for W/R/S), walk-away 0.40 sec, or spell cast, 0.40 (S)
If the monster doesn't do >= clvl damage (after mana shield reduction), no stun can occur.
Calculations are for warriors, melee rogues (Sword&Shield) and muscle mages looking to stay and swing, for Battle Mages looking to cast, and Arty Mages looking to flee.

RESULTS
I) Monster hits Player 100% of the time (pre-block, using base To-Hit, bonus and player AC)
Class Benefit of Harmony? Benefit of Fast Block (FB)?
W: Doubles #monsters can face before stunlock. No use -- automatic for warrior.
R: 75% increase in #mon can face before stunlock. Doubles #monsters can face before
Also allows walkaway from one lone attacker autoblock is broken
S-MM: 60% increase in #mon can face before stunlock Triples #monsters before block broken
S-Arty Even with harmony will die to stunlock Hope you can beat him up, FB no help
S-Tank Shouldn't be getting hit 100% :-) Rely on AC, not FB, to get away

* when Warrior hit only 75% instead of 100 %, harmony will keep him from having to run to a corner to avoid stunlock. At 75%, most hits are blocked by W/R/S, even in a crowd

II) Monster hits Player 50% of the time
Class Benefit of Harmony? Benefit of Fast Block (FB)?
W: Doubles #monsters can face before stunlock. No use -- automatic for warrior.
But with good block, only relevant if surrounded
R: 75% increase in #mon can face before stunlock. Doubles #monsters can face before
Also allows walkaway from *two* attackers autoblock is broken; hit unlikely
S-MM: 60% increase in #mon can face before stunlock Triples #monsters before block broken
Like War&Rog, very few actual hits
S-Arty Can escape a lone attacker w/ or w/o harmony No- running, hoping next swing misses
S-Tank Can escape a lone attacker w/ or w/o harmony May allow spell cast after block

* when Rogue hit only 38% instead of 50 %, harmony will keep him from getting stunlocked even when surrounded.
* when Sorcerer hit 30%, harmony will avoid almost all stunblock, even teleporting into a pack

III) Monster hits at the Autohit level (15% usually, up to 25% or so in deepest hell)
W: No problems with stun, harmony not needed Auto FB
R: At this low hit rate, harmony & FB only help the rogue dish out damage with less time interrupted, neither is critical anymore
S-MM: Harmony will still help avoid damage FB not crucial when hit this infrequently
S-Arty You're no arty if only hit by autohit! Not useful
S-Tank Will still help when surrounded by >=5 Better uses for a shield than FB here

CONCLUSIONS
1) Warrior
If hit >= 75% of the time, harmony very useful Auto FB, no special shield needed
2) Rogue
If hit >=38% of the time, very useful Hit >=38% very useful, hit <22% not necessary; hit 22-38%, difference between immune/non-immune
3) Muscle Mage
If hit >= 30% of the time, very useful Hit >=30% very useful, hit <22% not necessary; hit 22-30% is important only when really surrounded
4) Artillery and Tank Mage
If hit>80%, no help; 60-80% allows escape; Hit very often or infrequently, not 40-60%: harmony is some help, 15-40% it will helpful, Hit 33-75%, will reduce damg increase #monsters you can walk away from and allow more spell casts

SPECIFIC EXAMPLES
Table of AC's required for 40th level character giving monsters various ToHit chances...
Location Autohit 50% 75% 100%
NM/H 230 195 170 145 DEX for 100% block = 220 - 2*clvl
H/cave(IW) 191 156 131 106 So block% is DEX + 2*clvl - 120 (max 100)
H/H 295 260 235 210

Ex.1. 40th lvl Arty Mage with AC 85, and 90 DEX, will be hit 100% but block 50%
Will get almost no help from harmony or fast block and will be slain by an undetected illusion weaver

Ex.2. 35th lvl melee Rogue with AC 275, and 200 DEX, will be hit 45% but block 100%
Will find harmony and fast block to be very helpful, but not as necessary in 5 lvls or with 20 more AC

Ex.3 30th lvl Tank Mage with AC 225, 100 DEX, worried about Azure Drakes and Illusion Weavers. Hit about 100% and 15% respectively. Hit too often by Drake for either Harmony or Fast Block to help much, and too little to bother with them against Hiddens

Ex.4 49th lvl Muscle Mage, Charles wonders if it's time to try harmony or Fast block. AFP Precision gives great AC and DEX. Too late in life to be off value. But at lvl 33, with AC 240 and good block skills, would have found it indispensible.

Ex.5 Cathrin[BWO]'s 35th lvl Ultra Pure rogue wears Stormshield and moderately good equipment. 100% block but only 240 AC. She finds an Emerald Shield of the Tiger and a Jade Amulet of Harmony. Running H/H, can she ditch SS? Replace Obs./Stars Amulet? Table shows she will be hit up to 80%, recommending fast block is crucial (keep SS), and harmony worth investigating. Why? The harmony would allow her to face almost four instead of two opponents before being stunlocked.
What about earlier levels of hell, against lava maws, which hit 75% less than Blood Knights? Basically, this is in autohit range, so 15%. So if Maws and Witches instead of Knights, swap out Harmony and swap in Emerald Shield :-)

KNOW YOUR HARMONY ZONE
Since the need for harmony is SO dependent on class, stats, ToHit, AC and opponents, there can be no universal answer as to whether it's needed. So for *your* character with their favorite setup, look at your AC and for each of your favorite 'hunting zone' (ie NM/H, Hell/Cave)... check the tables above to see if harmony is of benefit there. Double check to see what creatures have fast swing rates and if they even do enough damage to stun you, and you now know your 'harmony levels' for that character. For mages it could be a large part of the dungeon :-)

IMPLEMENTATION
So if your characters' current class, lvl, opponents and gear suggest that harmony and/or fast block is helpful, how do you get it? Fast block is obvious, as it mustgo on the shield. Chances are that will displace a suffix of +all, +stat, or +hp. But what about harmony? Choices are: jewelry, armor, or helm. Now since the best helms are uniques (no harmony), and jewelry can go up to zodiac while armor can go up to stars, the armor is the logical spot for harmony. It's benefit, when indicated, should offset the hp or stat bonus. In practice, however, there are problems.
For low level characters, you take what you get, so if you find harmony on a nice base item with a prefix to boot, go for it. High level characters are likely to want to wear Awesome Full Plate. The chances for AFP Harmony are pretty low, but consider Glorious FP Harm and Blessed FP Harm (buyable at Wirt) as the item to swap in when you want harmony. More likely you will buy highest AC plain AFP you can, and will consider swapping in a Harmony jewel for a +all. How does Harmony compare with Stars, Heavens and Zodiac? For a warrior, +10 to all would give +10 mana, +20 hp, +10 Dex, +2 AC and +5 ToHit. The warrior wants harmony when he's getting hit 75% of the time, so being in stunlock half as much is worth (IMHO) the extra hp and to hit advantage. For the rogue, dex is high enough that to hit and blocking won't suffer, so unless that jewel made the difference in whether armor could be worn or not, it should be swapped in when on 'harmony levels'. For sorcerers, each +10 to all would equate to +20 mana, +10 dex, +10 str, +2AC. But for them it's more binary: no trouble or stunlock-death-candidate. Learn what enemies are in your harmony zone, and swap a jewel (or armor) into your setup on those dungeon levels.

Well, that was long, my thanks if you made it through. I hope this was helpful, and I look forward to comments/corrections/anecdotes :)

.... Appendix -- Swing Rate table from Jarulf's Guide... Monster Speeds           
Monster type       Walk speed  Hit recov  SwingSpd  Hit time
Zombies            1.20        0.30       0.60      0.40
Fallen Ones-spear  0.55        0.55       0.65      0.35
Fallen Ones-sword  0.60        0.55       0.65      0.40
Skeletons          0.40        0.30       0.65      0.40
Skeleton Archers   0.40        0.25       0.80      0.60
Skeleton Captains  0.40        0.35       0.60      0.40
Scavengers         0.40        0.30       0.60      0.35
Winged Fiends      0.65        0.45       0.50      0.25
The Hiddens        0.40        0.40       0.60      0.40
Goat Men           0.40        0.30       0.60      0.40
Goat Men Archers   0.40        0.30       0.80      0.65
Overlords          0.50        0.30       0.75      0.40
Gargoyles          0.70        0.50       0.70      0.35
Magma Demons       0.50        0.35       0.70      0.20
Horned Demons      0.40        0.30       0.70      0.35
Spitting Terrors   0.40        0.40       0.60      0.40
Lightning Demons   0.40        0.20       0.90      0.25
Balrogs            0.35        0.05       0.70      0.40
Vipers             0.55        0.25       0.65      0.40
Succubi            0.40        0.35       0.80      0.50
Knights            0.40        0.20       0.80      0.40
Mages              0.05        0.30       1.00      0.40
The Butcher        0.40        0.30       0.60      0.40
Skeleton King      0.30        0.30       0.80      0.40
Diablo (Diablo)    0.30        0.30       0.80      0.20

Short Swing speed times, or multiple attacks within one swing cycle are what to watch out for, to see if you're stunlock bait.