Sword main = average PvE, average PvP, pretty average 'tank' main. Nothing really special and outstanding frankly. Requires a VERY high level of skill to excel in PvP.
Spear main = average PvE at start, decent and strong PvP when used in conjunction w/ spear merc. Very quickly becomes one of the most versatile classes, able to fit into any formation. Primary skill Lightning Spear never loses value.
Gun main = strong PvE, strong PvP, generally has to specialize toward having multiple other range mercs, but a very solid, easy to play main. Main downside is that the primary skill (Wild Shot) becomes useless VERY quickly. However this is remedied by lvl100 with the new skill Scorching Bullets.
Cannon main = very strong PvE, strong PvP if specialized toward a stun build. Extremely easy to play and fits well into any 'bombsquad' or ranged formation. Primary skill Deep Insight never loses value. Very fast levelling class.
Staff main = Extremely strong PvE, strong PvP, one of the best PvE classes in game and one of the fastest levelling due to AoE nuke spam. Very versatile class, can opt to play either primary healer, or offensive nuker, fits into any formation.
Archer main = average PvE at start, very strong PvP. Arguably one of the strongest PvP classes, if not the strongest, as of the last patch due to critical formula rework. In PvE, primary skill Silence very quickly becomes useless unless in PvP. Class improves GREATLY at 100 when Arrow Cascade is gained. It's proverbially a class that sucks all the way until lvl100, then it becomes one of the best in game.
Axe main = Decent and easy PvE in the beginning, decent PvP in the beginning. Very quickly becomes a totally redundant main overall as the value of primary skill Freezing Axe is totally outdone by the northern viking's Ice Ridge Axe. Lvl100 main upgrade skill is total freaking WORTHLESS. General consensus on official forum: If you made an Axe main and are still under lvl30, reroll... If you're over 60-70, buy a Vial of Amnesia and change class...
Musician main = Strongest PvE class in game, strong PvP even after recent nerfs. It's a pure Damage Over Time class, which means you must play specialized toward that. Can potentially get dull because of it but is a very good class overall. Strong in PvP, but DoT style is largely made useless due to the double monk fail going on.
Powersaw main = Requires a lvl100 character on the account to even create. Has all the benefits of an Axe main, with NONE of the downsides. Primary skill is a strong AoE attack. Lvl100 skill is as useless as the Axe main's skill, but the class overall has more than enough positives to make up for it. Storm Blade, it's 2nd skill, was recently changed from an 'activation' skill to an on-hit proc, making it MUCH better. Overall a very, very powerful class, the only class leaning towards 'imba'. Considering that it requires a lvl100 main on the account to even create however, it can have it. It's powerful but not brokenly powerful, the balance is still very solid.