Okay, here are my impressions after playing for about 12 or so hours online.
All of the classes all seem to be pretty good. I had the gamestop unlock preorder code, so I got to try all classes with all of their abilities unlocked.
Engineer
First thing I noticed is that the sentry turrets have a nice damage boost from KZ2. I got quite a few kills from both the rockets and the gun. That sentry rocket is pretty wicked if the turret is placed in a good spot. You simply cannot ignore a turret if it is firing at you youll die. The engineers level 3 gun is, of course, pretty uber. You cant look down the sights, but if you crouch, if can mow people down. In CQB, forget about it its a buzzsaw. I had a lot of fun playing this class, and I HATED Engineers in KZ2. I will be putting a ton of points into this class after inf & tactician because its a pretty fun class that can lock down a particular area/avenue of attack better than any other class.
Tactician
I felt most useful to my team when I played tactician. There were a few games that I KNOW we won because I captured a key TSP. The M82 rifle is pretty damn good and the best assault rifle imo. Amazingly enough, that stupid Recon drone got me quite a few kills. I forgot about it a lot though. The rocket launcher secondary weapon is great and very useful for taking out walking mechs and for clearing out a TSP. I start almost every game as the Tactician class.
Infiltrator
Sab was my favorite class in KZ2, by far, and its even better this time around. The shotgun is ridiculously powerful (which is a plus and a negative, depending on which end of the blast you are on) and the SMG and Assault rifle are pretty powerful as well. I usually opt for the assault rifle so I can kill from a distance. Of course, its a lot harder to tell that an infiltrator is an enemy in this game, and I definitely fooled a ton of people. You just have to play a certain way youd be amazed how something as simple as walking backwards fools people. On that map where it is snowing, I went wild and had a 12 melee kill streak before I was finally gunned down. Tons of fun. I definitely had my best K/D ratio with this class (I dont snipe), and the class is really useful for S&D and assassination modes.
Marksman
I spent the least amount of time as a Marksman because its not my style of play, but I sure was killed by a shitload of them. This was easily the most popular class, and I guess I understand why. You can get cloak and carry an assault rifle and a silenced machine pistol at the same time. On one map, this marksman went something like 45-1, and that death was by my hand as I stopped everything I was doing hunted him down for 15 minutes. I dont complain about anything being OP, as everything has a counter, but being able to kill from cloak with the machine pistol and remain cloaked is more than a little annoying. The thing is is that these guys dont do much to help their team to win (imo), but they dont care going 16-2 is their goal. Its annoying when Im facing them or when they are on my team, but thats how I view snipers in *all* games. The maps are great for snipers though plenty of good positions to camp with your rifle and snipe away. Scramble++ ability is good you dont realize how important your map is until its scrambled. It makes me a little paranoid when my radar gets scrambled because I know the marksman can kill me whenever he wants with his silenced machine pistol. Annoying but not game breaking or anything. Very useful to have on the front lines if the marksman doesnt sit and snipe. I suspect that many organized teams will employ a marksman as a front line fighter armed with an assault rifle.
Field Medic
I wish I could heal. I liked how, in KZ2, you start at full health but when hurt, you only heal up a certain percentage. That made medics even more useful, as every bit of health helps. No idea why that is gone. As everyone will say, the revive gun takes way too long to recharge. Further, I lost count of how many people tapped out instead of waiting for me to revive them. With mics and better teamwork, it would help, but there really should be some type of indicator that a medic is nearby or, at least, allow the dead guy to rotate his camera to see if anyone is running to assist. I also did not like the medics assault rifle or SMG SMG seemed too weak (though I saw some others dominating with it) and the assault rifle needs to have full auto as the burst made it seem too weak as well. Triage++, the ability that allows you to resurrect from the dead, is a great ability. I was able to kill my killer quite a few times. Overall, though, I just didnt feel that useful to my team as a medic, which is a shame because it was my second favorite class in KZ2.
Unfortunately my unlock code has expired, so its back to leveling up for my uberness.
I know some people complained about raising player health, but, judging by the number of assists people get, I dont think its an issue. As for the maps, some are pretty bad but Im withholding final judgment until I get to experience them with the GAF clan. I have a feeling that teamwork will make everything a LOT better.