I think the worst game I bought (somewhat recently) was Pirates of the Caribbean for the Xbox. I read the reviews, read the problems, but heck, your a pirate, it's like Morrowind (a game I loved), how can this be bad? After 3 days of playing, I started playing Morrowind again, at least it was only $20.
For the guy that bought Superman 64 at launch, I might have you beat (or close). I bought Rebel Assault II for the Playstation for the full $50. I actually enjoyed the game, but here's the kicker. It was between Rebel Assault and Panzer Dragoon Saga for the Saturn. I decided I didn't feel like an RPG, I'd get it next month I want a game. Needless to say, I can buy Rebel Assault today for $1 or $2, Panzer, if I wanted it, is far far more.
Edit: Earlier in this thread, there was lots of dissing of State of Emergency. I know it was hyped, but I bought it for $20 and never regretted it. I, to this day, will still pop the game in for friends to let them play Last Clone Standing. Playing 2 minute rounds of blowing away 200 people, it's lots of mindless fun. Is it as good as the GTA's, not even close. But, on it's own, I think it's an ownable game. Plus, all my friends who play enjoy it for what it is. Even non-gamers enjoy it. I think people are way too hard on this game.