Maining 1 class? Good or bad?
Bad...definitely bad. You get a lot of bonuses from playing all the other classes. Not only do they give you use certs to help make your character stronger, they also increase your stats passively...things such as your atk, hp, and recovery amount. So yeah, play all the classes...they make your character more stronger overall. Plus the game is designed where you should have multi main classes...I mean why else would they put a change-on-the-fly class system =p.
Mmm...staying stealth in pvp is a double edge sword. Often times the skills that make you go invisible in game, makes you either 1. walk slower or 2. take 2x to 4x more damage. Meaning if you walk into some AoE or get caught by an AoE...you are most likely going to get one shot.
Currently the strongest class in game is Mimic. Its a create-your-own-class sort of deal...allows you to use different skills from all the classes on that one class. So you don't need to worry about what class to main or what not now...because at end game, you can use whatever weapon. whatever armor, and whatever skills you want and still be very productive.
Just have fun, play the game, explore the classes, and lastly try to join a very high ranking guild and see what you can pick up from them. The rest will come naturally