I was cautious about the multiplayer because I was a firm believer that Mass Effect 3 should have only been a single player game. I loved the single player (the ending didn't detract from my enjoyment, as I am blaming EA for the ending), and the multiplayer is very addictive.
I use my Sentinel Human Male, he's at Level 17 at the moment, but I also have a Level 11 Adept Drell, and My Level 9 Engineer Human Female is coming along nicely.
Sentinel - Assault Rifle and Sniper
Adept - Sniper and Pistol
Engineer - Assault Rifle and Shotgun
I don't like the character unlock system. It would have been a lot better if you got the new characters as you level up that class, instead of the lucky dip which are the Packs you have to buy. I do not want a Quarian Infiltrator, I want a Turian Sentinel, dammit!