The most fun you'll have in any shooter, period.
The main highlight though is the Skillshot system, somewhat reminiscent of Star Wars Force Unleashed where you could perform crazy tricks with special names. With Bulletstorm, the more insane your Skillshots, the more points you get for buying ammo. There are so many amazing Skillshots in the game that the average player is not likely to pull all of them off in one playthrough, which invites multiple playthroughs and repeated attempts at the Echoes trials. Skillshots are nothing short of pure joy, from kicking an enemy into a cactus up through driving a spinning drillbit through a boss's stomach, they never get old. The Skillshots are the "Oh, Snap!" moments you end up telling your friends about later. Having a whole game dedicated to this concept is nothing short of incredible fun.
For some reason there is a tinge of hatred to this game due to fanboys being upset that it came out around the time of Killzone 3. This is very unfair to the quality of this game. Everyone should give it a chance, as it is a whole lot of fun, and worthy of every gamer's attention... even those who normally do not play FPS games.