I did indeed have the displeasure of using this enchant.
1. The proc rate is insanely low... 1ppm maybe less
2. The effect really doesn't do that much even in the best case scenario where everyone gets healed for ~250 health.
The sad thing is they have to keep it this way to balance it with fury warriors. I think on the PTR they had the ppm a lot higher, and dual wield fury warriors with blood craze and blood thirst, had it proccing so much they could continually kill equivalent and -1 level monsters at 70 without stopping at all. So in order to balance with fury, us tanks get the shaft =T
Situational at best, but has a few uses. I put it on an extra weapon just to mess around with it. It does heal pets, it will proc 2-3 times in a row, but overall lower proc rate. The times I do use it involve some sort of aoe or random damage, like the Aran fight where aggro isn't sensitive. By far the best use I found for it, is the Netherspite fight. If you use the two tank red/green beam phase rotation, in the green beam battlemaster will go off for 400-900 heals on your party. Not game breaking, just a boost to help with the aoe damage since aggro on Netherspite is pretty much a non-issue.
I've had Battle Master on my King's Defender for about 2 weeks now to test out its effect in raids and such and really I found it to be very lackluster for tanking threat wise. It would tend to proc once per minute and generate about 1k threat but that's so minimal compared to a good move rotation you can pretty much negate it completely. I recently just switched it to Mongoose like on my Sun Eater and It's working out just fine.
All in all Battlemaster isn't a great tank enchent or enchant in general unless the entire group is all melee and all have it, even then it's still a crap tank enchant.
Its about 1ppm, and with devastate you end up getting about 1.5 ppm.
The problem with it is that it ends up doing about 60-80% overhealing. It healing is completely unreliable for survival purposes, its only good for sustained threat over the course of a boss fight.
So about 1.5 times per minute you generate an average of 240 healing (180 threat in defenseive stance with defiance), on about 2 people. Thats about 540 threat per minute, or under 10 TPS.
Mongoose's crit and haste effects tend to generate more threat than this, and its armor and dodge prevents more damage than battlemaster heals, and is more reliable. Mongoose is massively better.