Games worth the money:
- The Orange Box (every game separately too, but the collection even more so)
- Fallout 3
- Dragon Age: Origins
- Civilization V (and likely every one before it; though upgrading isn't really worth it)
- SimCity
- Altitude
- Machinarium
- World of Warcraft (if you're into that sort of thing)
- Minecraft
- Flower
- Battlefield 2
- Battlefield 2142
Games that aren't worth the money:
- Battlefield: Bad Company 2
- Age of Conan
- Warhammer Online
- City of Heroes
- Spore
- many others whose names I can't remember anymore
As far as I am concerned, duration has little to do with the value of a game. Portal, Machinarium, and Flower are some of the best games I have ever played, even though they lasted less than 8 hours each. On the other hand, Age of Conan was the worst experience I have ever had and it probably had over 100 hours of gameplay. It's about quality, not quantity. Portal and Flower were wonderful experiences which were worth the $10-20 that they cost. In all of its hours, AoC never provided an enjoyable experience.