It really depends on you. The guidelines I like to use are, "Will it benefit in some way (e.g. learning or inspiration)?" and "Do I have the willpower to prevent myself from going overboard with it?"
In my case, Spore sounds like a dream game and I will definitely be picking it up, but I may have to employ some harsh methods on the willpower side.
Just be careful, and remember that there's absolutely nothing wrong with taking a "break from life" and enjoying yourself once in awhile.