Well this is going to be disappointingly long list. I'm a combination of a D&D/d20 -nut and a mad collector, which has made me buy many things on a whim. Oh well...
Books that I really regret buying:
Deities & Demigods -- I wanted rules on how to create stats for gods. I never found them.
WotC Splatbooks -- For all the reasons already mentioned.
Arms & Equipment Guide -- Uninspired. I will get some use out of it, but I was very disappointed.
Stronghold Builder's Guidebook -- I'll never need this one.
Hero Builder's Guidebook -- Why do I keep doing this to my wallet?
Psionics Handbook -- Nothing against psionics as such. The mechanics were quite unbalanced though. I hope the 3.5 update fixes this.
d20 Wheel of Time RPG -- Hate the setting. I was looking for interesting mechanics.
d20 Star Wars 1st ed. -- They should have gotten it right the first time. I guess they were training for 3.5.
Ravenloft d20 -- I don't know why I bought it. I didn't like the setting before, I don't like it now.
Divine & Defeated -- Horrible mechanics.
Ghelspad Gazetter
Hollowfaust
Mithril: City of Golem -- Scarred Lands wasn't my setting after all. However, the above products are ok for those who like SL.
Encyclopaedia Arcane - Battle Magic
Encyclopaedia Arcane - Necromancy -- Why Mongoose publishing cannot bother to learn the d20 rules?
Mythic Races -- I have gotten absolutely nothing useful out of this book yet. Maybe some day...
Books that I love:
3.5 core books -- Overall a great update with minor wrinkles here and there. I can iron them out.
FRCS
Magic of Faerun
Races of Faerun -- I like the Realms... well *my* Realms.
Relics & Rituals -- I love the ideas and the tone of the book. I can fix the mechanics with minimal effort.
Book of Eldritch Might I & II
Book of Divine Might -- I like Monte's stuff. Most of his spells and Prestige classes are great. The alignement variant in BoDM is very nice.
- DJ