No real direct personal experience (not counting a few DIY experiments that I can't quantify results of), but from my reading of many others' experiences, I'd say 50/50.

Seems to depend highly on the particular battery and the conditions under which it became sulfated in teh first place, how long it's been that way and what exact type it is (SLA, FLA, AGM, deep-cycle, starter, etc).
Doesn't necessarily matter which kind of desulfator, either, although the electrical type that uses high voltage pulses seem to be a mite more successful wtih SLA, as they dont' usually cause venting that will quickly make the SLA useless (unless you figure out how to refill the venting electrolyte).
I do think (based on physical behavior seen in various chemistry experiments rather than experience with desulfation) that doing some thing that keeps FLAs stirred up will increase the chances of it working, especially methods that involve vibration as that may help break loose crystals from the plates to allow them to redissolve into electrolyte more easily.