Everything about the game, from the environment, the driving, the animation, the characters, the graphics, even the character switching (except it sucks that they change their clothes), is awesome. Everything except the missions.
The only missions that end up being a little challenging are the ones that are like linear TPS missions. They aren't good like in old GTAs where you could invent new ways to beat the mission like set up cars to make a blockade, or steal a helicopter to use for the getaway, or buy some explosive weapons first to help you. Instead, you get the C4 automatically and simply put it on all the dots and beat it on the first try. Yeah the driving is great, but you can only really use it to fuck around and rip through pedestrians since there's no tough driving missions.