The missions are all pretty straight forward. You are free to roam around and collect DNA, but I got a bit bored after ripping out my 1000th brain from a yokel. Destroy all Humans also has some mini-games scattered throughout the levels, but they are all relatively similar, and each game only results in getting more DNA. It would have been nice to be able to upgrade in the field as opposed to just when you're on the mothership.
The sound in the game was well done, but the soundtrack, like the jokes, begins to wear on your nerves after a time. I think that the gamels biggest success was definitely the slick graphics.
I did find that the game has some limitations. I created a large chaotic scene, and had many cars exploding on the screen at once. This caused the game to shut down, because it can't handle you creating huge flaming pile-ups. I would definitely recommend this game for a weeks rent, but not much beyond that.