So I have the PHB2.
My impressions after flipping through it and reading the parts that appealed to me the most:
Chapter 1, New Classes: they are all perfectly usable. I really like both the Knight and Beguiler. They fill great slots in the base classes array of D&D.
Chapter 2: Expanded Classes: the fluff presents good stuff to read and reread when searching for a character concept. The alternate class features are sometimes "blah", sometimes good (spontaneous domain spells casting for clerics). I sure would like to have seen a few more per class, but these can easily serve as houserule examples.
Chapter 3: New Feats: Wow. So cool! Why don't we get more of these in WotC supplements? These certainly add a new layer of options for PCs (particularly in terms of tactical choices, if you ask me, for feats such as the Spring Attack chain, flanking bonuses and such).
Chapter 4: New Spells: Didn't have a good look at this section.
Chapter 5: Building Your Identity: Good stuff there. Certainly a good section to review before polishing a character freshly generated, or just before to get ideas for character concepts, this may help as a check list to make a character more than two dimensional with suggestions of backgrounds, personalities, traits and a few suggestions of table rules to smooth the gameplay.
Chapter 6: The Adventuring Group: Very good stuff in there to think about the different possible group configurations and how individual characters work together to achieve a good efficiency. I particularly like the suggestions of group compositions if some classes/archetypes are missing, you only have three players and such.
Chapter 7: Affiliations: So so there. The whole idea and its presence in the PHB2 is rather cool, but I'm not a big fan of the treatment in terms of game mechanics there. Affiliation Scores and such are a bit two-dimensional to represent groups and factions in the game. I think they're better represented by prestige classes, but why not have both, really? I'm sure it will inspire some players and DMs.
Chapter 8: Rebuilding Your Character: This is really a cool part, from how to rebuild a characters' stats to how to implement the possibility of rebuilding characters in particular games with examples. How to take the rebuild as an opportunity for quests and encounters in-game. Nice.
Appendix: Quick PC and NPC Creation: I love this part and it will be both useful for me as a DM and player. Want to generate your starting equipment automatically for any character level and class? It's there. Want examples of feat selections for every class and each, with different examples of what to select for what kind of role you want to fulfill in the party? It's there. Want help to select your spells? It's there.
This kind of advice was long overdue, in my opinion.
Overall? Great book. Most definitely. The classes and feats will see some use in my games, that I can assure you. It's a bit too early for me to have an opinion on the spells proposed therein.