My favorite creature to play as is the Alien, so fun running on the ceiling and jumping onto unsuspecting humans.
The Alien is the one that I can't get used to. It's fun to play as now and again, but unlike the other two, I often feel weak as the Alien. The lack of range is a bit annoying, and the Alien's heavy attack doesn't seem as good as the Predator's.
I just had three good games today. Played a 1v1 as the Marine against a Predator and beat him 5-1, then played as a Marine against 1 Predator and 1 Alien and beat them too (although the Predator left about half way in). Finished up with a good game as the Predator, beating another Predator (who also left half way through) and an Alien. I've won lots of games now as the Marine and Predator, but still haven't even come close as an Alien. I can't get any good killstreaks as an Alien.
Species Deathmatch - 4 Predators v 7 Aliens v 7 Marines (I think)
I think it's supposed to be 4 Predators, 6 Aliens and 8 Marines. Not entirely sure on that though.