PvP is pretty fun. There are level-based, free for all battlegrounds where you can go and just wail on other players.
There are also more coordinated GM-run PvP events where teams of players can face off against each other or where a whole bunch of people fight a GM.
Obviously, players who have cash shop items (HP extenders, potions, etc.) will stand a better chance against those who don't, but it's still fun to get a group of friends together and fight some people.
For starters, I think that cleric is the best route. The classes are way unbalanced, but I still think that cleric is the best way too go. I chose fighter but when I first started the game, I did ask for any advice or anything....so basically , I knew nothing. But like i said, class wise? Cleric all the way ;D