Hmm, what do I regret?
There are a few that I truly regret and just had to get rid of:
Creature Collection and Relics & Rituals. Creature Collection was one of the first d20 books I bought. I wasn't immediately disappointed, but soon it just began to weigh upon me like a stone. The mechanics weren't there and there was too much Scarred Lands 'embedded' into the book. Relics & Rituals was better but I found very little useful. Both were sold early on.
Arcana: Societies of Magic. I don't normally dislike Green Ronin products but this one I could have done without. I was hoping for something more along the lines of the old 2E College of Wizardry sourcebook (which I still own and won't give up... period).
Sword & Fist. Just a bad, error riddled book. (On a related note, I bought both Tome & Blood and Defenders of the Faith and am happy with both books.)
On the chopping block:
Oathbound: Nice book, but I haven't any use for it. [ Sorry Ashy.

]
Arsenal: Bought this to go with Dragonstar. I wish I hadn't. The weapons in it are just too 'out there' for me.
3.0 PHB, DMG, and MM: I've moved to v.3.5e... nuff said!
As for the current books I own that I'm on the fence about are:
Deities and Demigods: I was really excited when I bought this book and read through everything up to halfway through the D&D Pantheon. Then I sort of lost interest in it. I dunno, I think I was hoping for more 'background' religion material.
Miniatures Handbook: Ok. I'm only on the fence about the second half of the book. I bought the book second hand for only $20 CAN, which is damn good considering it would have cost me $40+ CAN brand new. I love the new base classes but have yet to become excited about the new prestige classes. Must wait and see.
Oriental Adventures: I need oriental rules but I hate all the Rokugan stuff, which is 'embedded' into the book. I will likely hang onto it for the basic rules mechanics such as base classes, feats, and monsters.
Monster's Handbook: One of the best FFG books I never seem to have time to use. The book is great but I just can't get excited about it.
Psionics Handbook: I really like the concept of psionics but I have trouble with this incarnation of the Psionics Handbook. The book feels incomplete to me. I'm really hoping the new Expanded Psionics Handbook is as good as quality as the new v.3.5 core rulebooks. The book needs to be THE BOOK for psionics - like a psionics PHB, DMG, and MM combined.
Stuff I can't do without:
New Core Books: The best incarnation of D&D I've seen since the Original 2nd Edition version.
Complete Warrior: Ok, this is best splatbook Wizard's has ever put out. It is what Sword & Fist should have been. Even if the rest of the series bites. I'll have lots of use for CW.
Manual of the Planes: I'd chew off my right arm before I give up this book.
Monster Manual II: Better than the original 3.0 MM, IMO.
Savage Species: I'd be lost without the tables in the back, and I absolutely dig the Anthropomorphic Animal Template. (I think it is this book, which has made me less interested in the Monster's Handbook from FFG.)
Sorcery & Steam: After buying this book, I brought it home and read it cover to cover. Now, I have a basis to truly begin to build my homebrewed Time of Ages high-fantasy/steampunk campaign setting.
Honorable Mentions
Book of Exhalted Deeds
Book of Vile Darkness
Epic Level Handbook
GG's Necropolis
Mythic Races
Seafarer's Handbook
Spells & Spellcraft
Books I own that make me giddy.
Book of Templates: Deluxe Edition: Ok, while I'm a little biased (see sig), this book is one of the best PDF products I've had the pleasure to read. And I read it, from beginning to end, before it was finalized. Even the non-finalized version had my mind racing with new ideas. Hats off to Ian and the rest of the design team!
Draconomicon: Book of Dragons: To me, this is my D&D purchase of 2003. This book is fantastic and I've only read bits and pieces of it so far. Happy Happy! Joy Joy!
A Magical Medieval Society: Western Europe: Every gamer should buy a copy of this book. Trust me it is worth it. Can't go wrong with a ENnie award winner.