Some people may say that Crackdown only sells well because of the included Halo 3 Beta. I think differently however.
Crackdown's art style is the first thing that pops out, and it is amazing. With think black lines around everything, it makes it look better. The gameplay is very good. It is fun to just run around and do whatever you want. The main story (if you can call it that) features three gangs (Los Muertos, The Volk, and Shai-Gen) that you must eliminate from the city. That will only take the average gamer about 10 hours to complete. But the real value lies not within the main story (once again, not really a story involved here), but within the countless hours of stupid fun that you will have. This fun is doubled in co-op mode. You can make up your own "game modes", such as hide and seek (in which the seeker turns of their HUD and the hider, if you will, gets to drive right by the other person, laughing while the seeker looks for something right in front of him/her). And to top it off, achievements were made for this game. There are fun ones, simple ones, hard ones, and some that will require a lot of time to complete.