Crackdown
Comparisons to GTA fall apart quickly - while Crackdown has the open-world violence thing going, the gameplay really feels different. For one thing: driving sucks. Cars handle like hell, and since your bioengineered crime fighter can jump over buildings (and run faster than most cars drive anyway), I haven't even bothered trying to drive since my first couple missions. Also, the aiming is very different, using an accuracy mechanism that makes it more ... fun, like a minigame.
On the one hand, the game is definitely short. In two sit-downs (of a few hours each), I've already maxed two of my character's five primary stats, and cleaned the crime-ridden city of two of its three criminal organizations. On the other hand, these hours have been amazingly fun. Between leaping across rooftops to find agility upgrades, exploring Pacific City for new criminal hideouts, and just plain tooling around looking for thugs to mop up, there's (almost) never a dull moment.
The game is not without flaws; sometimes the aiming gets caught on things you don't want to aim at, sometimes a target is unreasonably difficult to find and takes some wandering around to get to. And like I said, it's definitely short. But man, is it fun.
Progress: Cleaned up Los Muertos and Volk