I've never been a fan of positionals in any game, with maybe the exception of backstab on rogues, and even that had to be addressed in a number of games to make it work from not-the-back, or situationally from not-the-back, or from not-the-back if you took talents/aa's from it, or "back" being a 270 degree arc.

Either the class is balanced around hitting every positional, which makes fights where mobs like to spin to cast on things a nightmare and you're better off using something less reliant on them, or its balanced around positionals being a bonus and those classes are by far the best option on any fight where you can reliably hit them. The best case being where you're expected to hit a certain number of them, and fights are tuned to where you -should- hit that many, but it still ends up favoring one or the other by a large margin.

If positional damage was calculated based on client-side timing, it would be significantly better, especially when things like to spin around and take their hitboxes with them, but would be easily exploitable. As it is, ive seen plenty of trick attacks fail from as directly behind a mob as you can be just because the server sees the input and goes "naaaaaaaaaah", so they seem to be more harm than good, and weve been seeing changes made to lower the impact of positional stuttering.