I rarely regret the games I buy, but there was some I bought that I wished I ranted or borrowed before hand.
1. Cursed Mountain: It was just bad, I didn't get far into the game, but what little I played I didn't enjoy.
2. I know everyone is going to hate me for saying this, but GoldenEye 007 for my wii. I had so much hope that this game would blow me away like the old one did, but the moment I started playing it, it just didn't feel the same. I played through a bit of the game, but I just didn't enjoy it like the old one. I still might have bought it later, but I regret buying it before I tested it out.
3. Kirby's Epic Yarn, the game was a cake walk. I chose not to finish it because of how easy the game was. I died once in the and that was playing multiplayer with Mike, who didn't know how to play the game at that time. After we played it for a bit, the game was impossible to lose at. So it's another game I would have still bought, but I wish I tested it out before buying it.
4. Pokemon Battle Revolution, no where near as good as Pokemon Stadium and for the first time ever when I cared about online game play saving this game, even that fell flat on it's ass.
The list would have been longer if i didn't sell all the games that I either didn't like or they disappointed me.