Along the lines of the thread "Games that won't let you go" - when I read that thread it got me thinking about the games at the opposite end of the spectrum: games which I still cringe at having purchased either because they were terrible or I just never got into them.
My first pc game purchase was Ultima IX: Ascension (6.4 at Gamespot) - and it has to be the worst game I've ever bought. Its box sits among all the other games I own and serves as a reminder of why I don't buy before reading reviews. I bought it on impulse after looking at the box in the store; it promised "complete immersion into the rich fantasy world of Britannia".
SWAT 3 although not a bad game, I just never actually played it beyond a couple hours. Which is a shame.