In general, yes. Better for some specs than others, certainly. I think it adds a different flavor than hit/exp/haste/crit. Mastery is a very specific type of power boost.
For example, my Destro Lock. A solid mastery proc can really kick ass when I'm FnB Incinerating a big pack of mobs, or help burn through big adds with a well placed Havoc Chaos Bolt. It's pure bonus for Aff Lock, and I have a blast with high mastery builds as an Elemental shaman.