As 3.xx recedes in the rear-view mirror, I've come to regard a good portion of my massive 3.5 library with a great deal of contempt.
Fact is, WotC was putting out at least one and often two hardcovers a month. They made a ton of money - sure - but there is no way - not even a smidgen of a chance that expansion material was properly play-tested. Not even close.
In the result, when it comes to 3.5, my opinion comes down to this:
If it's a DM book - get it. By "DM book" I mean:
Monster Manual II-V, Libris Mortis, Lords of Madness, Fiend Folio, Fiendish Codex 1 and 2, Draconomicon, Heroes of Battle, and to a lesser extent Frostburn, Stormwrack, Sandstorm, Dungeonscape....
All of these WotC books for D&D 3.5 are useful to a DM in varying their adventures, coming up with new types of adventures and posing new foes for their players. As a DM book? I think they all rock. Heroes of Battle is probably the most original and under-rated book in the entire 3.5 product line.
But the key here is that these are DM BOOKS. They are not Player books (or at least, not principally Player books). Because that's where 3.5 broke into a BILLION PIECES. The more classes, spells and items that 3.5 added to the system via player books - the more it broke into an unplayable state.
Spell Compendium? Utterly broken. The Complete Anything? BUSTED. Magic Item Compendium? Broken. PHB2? Skip it. Book of Nine Swords? Nope. Book of Exalted Deeds? Vile Darkness? No and no again.
Every time you see a new spell or new class for a player to use? Say no -- and say it loudly.
By all means - use all the DM stuff to change your game and provide a new experience to your players. But the moment you go beyond the Player's Handbook for player classes and spells, your campaign is just circling the bowl. It's just degrees of broketastic at that point. Worse, you will assure yourself that any published adventures or modules become instantly unbalanced in favour of the increasingly broketastic power your players bring to the table.
I fully expect there will be a LARGE dissenting voice on ENWorld concerning this opinion - but I am utterly convinced my view is objectively correct.
The best thing, imo, about Pathfinder is that it allowed DMs to escape the Reign of Error that the Spell Compendium and the Complete Series inflicted upon late cycle 3.5 campaigns.
I won't go back to that crap for any reason. I am, for the same reason, extremely hesitant to permit the forthcoming Pathfinder Advanced Player's Guide at our table.
I urge excessive and great caution in expending player choices beyond the Core rules. Change your game and its setting to make it different -- but resist the siren call of new classes and spells for the players. It's just not worth it.