Slayer - level 19
I only got to level 19 on my slayer but they are a very strong class, didn't have any problems with any quests and also go to do some PvP, the only class which nearly killed me was a warrior and he was 3 levels higher and from what I gather level makes a huge difference. Damage was good, PvE was easy and PvP was easy. Leveling seemed really fast, AoE was great on this class. Actually a zerker killed me in a low level duel, he hit me twice and I was dead.
Archer - level 14
Did a fair bit of PvP on this class and destroyed everyone I came accross, missed a lot of shots and being stationairy sucked but from what I gather this does change later on. I found it a pain in the arse to level with the lack of AoE and well this game being very AoE intensive. I know we get that paint spell, the kick spell and our super charge move goes through mobs but it was more difficult. Some damage if critting was nice but don't know. Not sure how this class will be later although it was cool enabling PvP then running next to someone and charging an arrow and smashing someone.
Zerker - level 8-10 can't remember
Didn't get to PvP with this guy but PvE was easy mode, most of the mobs I would 2 combo them to death, AoE like a boss and hit like a train. Not sure how much fun this class would be later on as I just feel it's very much a 1 trick pony and a relatively easy class to play but it's fun 1 shotting things, hope to level this a bit and try PvP with it later but every quest was easy, I think I'm going to clear the start zone in real quick time on this guy, the mini boss guy on the hill I killed in 3 hits, going to see what the boss in the dungeon is like next beta.
Warrior - level 11-14
Primal leveled this one but has a quick go and at this level they melt face, I was pew pewing people and mobs, great attack speed and had no issues with mobs.
At this stage I would say the Zerker is the quickest to level followed by slayer. I would also say Zerker is the highest single target DPS atm followed by archer. Slayer seems to be the most utility and mobile followed by warrior.