First off, I'm not going to bother translating directly from korean forum chats . . . like most forums world-wide they're filled with crap most of the time (note: not all of the time . . . just most of the time).
Secondly, I haven't played the game yet, and I won't bother to even if I can go through the trouble of gaining access to Korean servers. I want to wait and play NA version when it comes out here. I'll just play L2 and/or CoX meanwhile.
Therefore, everything that I'll be writing is my opinion derived from second hand sources available from the Korean aion forums.
Taking those caveats in mind, take everything that I write with heavy dose of skepticism. I do. It'll probably change by the time we get our version

.
Anyways after browsing through aionsource forums, most folks seem to think there is a massive imbalance with classes. After browsing through individual class forums in Korean forum chats, there does seem to be much *****ing and moaning about their classes . . . every single one of them. Even the sorcerers. Less moaning in templar and clerics sections.
What can we conclude from this? If you're the glasses are half-empty type, every classes suck except for templars and clerics. If you're the glasses are half-full type, most classes are balanced and need some tweaking. Here are some examples.
If you go to assassin forum you get a post saying among the melees templar>gladiator>assassin type of whining. If you go to gladiator forum you get many posts saying gladiators suck. However, if you go through enough posts to get to some detail, it's apparent that gladiators do not suck. In fact I can find some saying gladiators can out-duel 1v1 templars (w/ difficulty), sorcerers majority of the time, while losing to assassins majority of the time. Of course there are counter claims, but apparently gladiators do well against PvE with polearms (i.e. take on multiple mobs at same time aka fast lvling even w/o getting into groups) and do well in PvP with dual-sword. Most gladiator forum posts suggest not going against templars with polearms . . . they'll pwn you if you do. From this it's easy to see why there seemed to be a large percentage of gladiators playing in one of posts that was put up by eliott.
As you can see, this game is still in infancy, and people are still experimenting with how to make their class uber especially against other classes. Personally, I think it's a good sign that there is depth in strategy, and that it'll take some adjusting by the korean community. This is not to say that every class is balanced. Two exceptions seem to be rangers and spirit masters. Rangers got some love w/ last patch but I think it'll be a while before we know how that affects the dpm of rangers enough to make them viable or not. As for spirit masters, I think it'll take more than a simple patch to make them viable for 1v1 PvP. I think they'll be fine for massive PvP even in abyss (aka fortress sieges) since most of the fighting seems to be on the ground. Not in the air. As the game matures (aka chronicle 1 or 2 patch in the future like in L2), I'm sure they'll make aerial travel/combat more important, and that's when they'll probably fix aerial melee combat (probably sooner) and give spirit masters a weaker flying pet. This is just a conjecture on my part.
Anyways, just my rant.
