At around 235MB the demo contains the complete Seriousopolis Uptown level from the game. This level can be played solo, or over LAN or online by up to four people with cooperative play. The demo also contains all the weapons from the game (but you'll have to cheat to see them all), one of the game's vehicles and a good variety of enemies.

You can download the demo from Fileshack and it should give you a taste of what to expect from the full game when it's released next month.