This is how I look at it. Every adhoc game has been fun, because of the group. Many of the games don't necessarily stand on their own merits.

But lets be honest, theres little that you guys would run that wouldn't be fun for one session, including Rifts or D20. The question is is there enough depth and possibility in the mechanics to make that certain game worthwile to return to given our limited time and ability to get folks together. Which is what I mean when I say I wouldn't play something again.