Perhaps I was overly optimistic expecting a book of the overall utility of PHB2, but I admit I doubt I'll be using PHB3 anywhere near as much as its predecessor.
As you would expect from the PHB format, PHB3 introduces several new PC races including Githzerai, Minotaurs, Shardminds and Wilden; and several new classes - many of which have been previewed on D&D Insider.
Much as Primal heroes were introduced in PHB2, the headline power source for PHB3 is Psionics which gives us the Ardent (Psionic Leader); the Battlemind (Psionic Defender); the Monk (Psionic Striker); and the Psion (Psionic Controller). If you haven't seen the D&Di previews, the Psionic power source operates slightly differently from other power sources in as much as it has "augmentable" at-will powers rather than Encounter Powers. (Monk powers operate slightly differently having combined attack and move options.)
Other new classes include Rune Priests (Divine Leader - arguably a variant of a Strength-based Cleric with alternate class Features); and Seekers (Primal Controller - ranged control effects that function through the use of a ranged weapon).
Another new option for 4e is the concept of Hybrid classes - in effect creating a new class by combining abilities from two other classes. This is one of the parts of PHB3 I'm least comfortable with: an inexperienced 4e player may end up creating a very under-powered character while some other hybrid options can create characters whose abilities far outstrip those of a regular single class PC.
My main criticism of PHB3 has to be that Power Creep is definitely setting in - more so than has been apparent in previous 4e supplements. PHB3 includes too many examples of "broken" powers and feats - several of them good enough that they render earlier feats/powers obsolete. The problem is that in amongst all that there are also a lot of good ideas and additions to the rules and this leads to the problem of deciding which to keep and which to disallow: a difficult call for any DM.
Overall, I can't honestly recommend PHB3 without voicing a few reservations. There are certainly a lot of interesting and viable options in there, but also too many game breaking options for me to say it's a "must buy" book.