I'm a creature of impulse and immediate gratification.
It is not unusual for me to declare, "X is the greatest thing evar!", and then to have left it gathering dust a week later.
So, having invested about 6+ hours in Assassins Creed, I can tell you that is a fun and cool game. Well worth the price of admission. However, it... well, I'll let someone else explain it for me: http://blog.wired.com/games/2007/11/review-why-assa.html
Also, one of the guys on Penny Arcade talked about "Why people play video games", and it set me to thinking. I rarely complete video games, whether its story driven games like Baldurs Gate (didn't finish), or open ended games like Alpha Centauri (finished once or twice, but played a zillion times and just quit when it got less interesting). I play games for the right-now, immediate enjoyment of them. If I'm having fun right now, the game is worth it to me. As soon as I'm done establishing cities on an alien landscape, or killing soldiers in a medieval town, I'm ready to move on to the next source of enjoyment.