Hmm well as a long time gamer and collector of RPG's.. I cant say I really regret having bought anything.. but thier have certainly been some books I was gravely disappointed in or have found no use in.
Most Disappointed with: EPIC level handbook
(I had high hope for the above, but all it turned out to be was a powergamers toilet reading.. instead of providing more options for what high level characters can do and become its just all about combat and battering harder creatures...yawn).
Gamma World Handbook
(Mechanically it isn't bad, and it looks nice enough... but the thing that annoys me is that it wasn't just updated it was effectively re-made.. and as a result it no longer has the "Gamma World" feel to it, its more Sci Fi and less Post Apocalypse now).
Dragonlords of Melnibone
(Well reminded Psion

, yup this book I bought as I love Moorcocks Young Kingdoms, but this was too much of a poor attempt to straight convert and it didnt work at all...a real shame really.
Least Use From: Enemeies and Allies
(Probably just me but despite seeing the iconics statted which was nice the rest of this books NPC's etc.. are just a waste of time, no new rules, new PRC's, new items or spells... just generic NPC's of which I am more than capable of making my own).
Most Treasured Books: Forgotten Realms Campaign Guide (just beautiful),
Midnight (Fantastic Premise, and i love the art),
Mutants and Masterminds (revolutionised the Supers RPG imo),
Babylon 5 RPG and Fact Book (few too many show stills and no original art, but the book itself is great and based on my favourite show of all time),
Star Wars Revised (its Star Wars...nuff said),
Armageddon 2089 (A wonderful Mech based system for D20),
Quintessential Witch (nice new class with some great new rules),
Fang and Fury (Vampires done right).
Most Treasure E-Books: Everything Darwins World (I wrote some fo Metal Gods btw

, this is just PA as it should be done.. and leaves the new attempt of Gamma World in the dust).
Most disappointing Non D20 Game: Pretty much every Supers RPG I ever bought... TSRs Marvel was okay but had so many balance issues it was unreal... but in general until M&M by Green Ronin the Supers genre for RPG's had been a dead duck.
Also pretty much every book from Palladium (although they have some fantastic setting ideas Robotech, RIFTS etc.., and have awesome art, thier system is just plain broken and they refuse to acknowledge it, I still however bought msot of thier books, which I probably shouldn't have done as i helped to leave them with the delusion that everything was fine).
Most Treasured Non D20 Games: All my Battletech/Mechwarrior material,
All my Earthdawn books (the worst publiscised RPG in history imo, which is a true shame as it was also one of the best).