The only time they aren't worth the money I get rid of them fast. Sometimes if I feel they weren't worth the money I will trade them to goozex or sell them on ebay to try to recoup the cost. I honestly plan out a lot of my purchases for weeks/months ahead of release dates.
If I have fun the entire way, and don't actually want to play a different game enough to remove it before I finish it, it's worth buying.
I was pumped about the new Hitman coming out and I got it opening day, but then I just didn't really like it. I'm only like 3/4 of the way there now and I can't play it for more than an hour at a time without getting bored. That was not worth it.