Crackdown is a blast. It's seriously great fun once you get into it for a half hour and realize what's going on. Most people are really enjoying it. Some of the weapons are a blast when you get powered up and have a 40 foot blast radius on your rocket launcher. Picking up cars and hurling them is cool.
This game surprised me. One of the best games I've played in awhile. Might have lasting appeal, but it's fun while it lasts.
I will say this though, once you beat it you're probably done for the most part. It's going to be a great 10-15 hours for most to finish it, maybe more if they like exploring and doing all the races.
I think it's a great rental. Glad I didn't buy it. I just don't normally play through games again and the multi-player is limited.