PoM was really easy PvE'ing, and worked great in a group PvP. Didn't play it at 20+ tho.
Honestly, they all seem pretty well balanced, especially from a group perspective. There is a bit of rock/paper/scissors aspect, but it just as much influenced by situation. The ones I felt that was the weakest were Dark Templar and Necro, but I read people love the necro, again just not my kinda class.
I'm trying to get our group to give the PVP server a chance. If so that would be the following:
Me: Guardian
Saphyre: PoM
Mosthula: BearSham
Nikolaos: melee of some sort
Daoiid: was thinking ToS but not sure (he's a big one to convince, if he gets ganked too much I'll get pissed and log).
Amit: He changes his mind each day so dunno

Hyla/Desmo if he buy's it, not sure.
There are a few others too, but one will probably be in another guild, just wants to get in the same server. Not sure if the others are going to play yet or not.