I strongly advise
against purchasing this game. I played in the beta for a few weeks and, while the dev team means well, the "higher ups" that make decisions on when and how changes are implemented will destroy the game. The lattice/matrix system (an extremely huge change to the gameplay) was introduced four weeks before the official release of the game. The experience system was changed radically one week before release. Purchase Planetside now, and you will be playing a sub-beta-quality product and paying $13/month to test it.
Before playing Planetside, I played EverQuest (on a PvP server) for three years which is also run by SOE (just like Planetside). After 4+ years of the servers being up, PvP in EverQuest is still broken and SOE has made no commitment to fix it. However, in that time they've managed to release four expansions and raise the monthly fee. Go figure.
If you're looking for a good FPS that really utilizes team-work and is balanced, play Q3F (
www.q3f.com) or the Natural Selection HL mod (
www.natural-selection.org). If you want to play a watered-down zerg-powered lag-fest that doesn't even compare to FPSes that were released three years ago, but holds a lot of people, play Planetside.