I've had this game since launch, but just last night dug into the multiplayer (which is the main reason I bought it, really enjoyed the demo). Got all 4 classes up to level 5.
Scout - I guess I thought I knew how they were supposed to be played, but whatever I was doing was completely the wrong way. Getting it handed to me.
Scientist - Odd that this is listed as the class that everyone struggles with, and this is the class that I had the easiest time with. Playing conquest, just kept b-lining from point to point, stopping by on the occasional perch to rifle some baddies, then onto the next point, healing my friends. This class easily had my highest K/D ratio. WAY more importantly, it felt like I was effective and helping my team.
Leader - Average. Nothing great, nothing bad.
Soldier - Same as the leader.
Had a lot of issues with melee, but looking at everyone else I was playing, its certainly a "me" issue. People seemed to bust it out quick, where my guy seems to do the "Hideo Nomo" melee attack. I break even at best. If I do get it off I miss.
Overall, Im really diggin' it. There was a small number of players on last night, but I hope that the game has some legs.