I rarely purchase games that I'm not 100% about. Pretty much never to be honest. The only two games I've regretted buying were:
Demigod - Terribad release. Stardock may be my absolute favorite developer, so I assumed demigod would be amazing. It was....ok. Turns out stardock didn't develop it, just published it.
WoW - I had a lot of fun with this game, I'm not going to lie, but when I saw what happened with the first major expansion I realized how terrible the mudflation would go, and then saw how they started homogonizing everything in a policy of appeasement (that's right, I went there). So, while it was fun, the amount of time I invested in it, only to see everything start going down an unsustainable path made me pretty furious. It was like hiking to the top of a mountain for the beautiful vista, only to find a McDonalds and a Super Walmart there.