MP is definitely better than SP, but it's still kinda lacking.
Alien gameplay is much deeper than AvP2010, but also glitchier... wallwalk is a nightmare to use properly, as the controls often get reversed for no reason, camera glitchs, and the character will sometimes just drop down instead of transitionning to the next part. Not to mention there are stuff you can't wallwalk on.
The 5 different alien classes (Soldier, Lurker, Spitter (starting) Boiler, Crusher (map pick ups) are actually cleverly designed with enough pros and cons any player can probably find the one best suited to their playstyle... there are also multiple moves you can unlock to further tailor your alien (see
here).
Aliens are really fragile though, they get shredded in mere seconds by Marine weapons, so I think it's going to take a while before skilled enough players can showcase the true potentiel of the Aliens in MP.
Marine plays like you'd expect, melee deals almost no damage but is good for interrupting attacks, shotgun is really powerful (especially pump), and the Pulse Rifle works great, especially when fully upgraded. Battle Rifle is also a great addition, it's lethal if you got a good aim.
Gametypes range from Team Deathmatch (5 min rounds, switches sides), Extermination (basically Territories from Halo, or Crashsite from Crysis 2/3, Marines gotta grab areas while Aliens defend), Escape (Marines can't respawn, and gotta make their way through 2 gametype specific maps up to the evac point while being chased by Alien players), Survivor (Marines can't respawn, and gotta survive for 6 minutes against Alien players).
MP is decent, but it has its share of problems though... Interface is poor, scoreboard will make the framerate drop each time you bring it, matchmaking seems really random, as players from all over the world seem to be pitted against (played a game hosted by a Chinese player, although every other player in the game was European), there's no host migration so if the dude quits your game is basically over, unless you play with someone in your area the game will be laggy to the point Aliens can just run up to you and beat you to death even if you shot them 4 times point blank with your shotgun (which is supposed to kill them in 1-2 shots), private games have no options whatsoever, you can't alter rules or gametypes...
It's still a fun experience, especially if you get to play with friends/fans, and it's definitely more interesting than AvP2010's, but without a serious fix to the issues listed above, I think it will probably die quick.