I only got to play one match so far, but based on it I thought it was a lot of fun. I picked the Marine. Most people were aliens, and there was another Marine and maybe two Predators walking about. There doesn't seem to be any down sights aiming, but there's some heavy auto lock that leads you towards enemies, so it balances out. I'm going to try it out as the other two species later. Overall it has about the same weight as Modern Warfare, with the health/weapon pickups of Unreal Tournament. Good combo.
So far it's definitely on my radar. It also has three campaigns, and there ain't nothing shabby about that.