PS2 is the multiplayer-FPS equivalent of drugs. Once you get the hang of it and how battles flow (game has become much newbie-friendlier over the years), you've been spoiled for a lifetime. There's just nothing like it out there, any other FPS will not be as enjoyable anymore. The sheer scale of it will kick you in the balls and you'll stay there like a docile puppy asking for more.
Game has been actively developed forever. Dev team never stopped fixing, balancing, boosting performance, adding content and systems (even scrapping some altogether) since release, that is Nov '12! It has an "over 10 year dev plan" and a kickass community and dev interaction. It is also a stunner artistically and technically and daylight cycles add a lot to that.
Frequent patching does break stuff often, which soon enough gets hotfixed. Also the game might get kinda stale after the first 1-1,5K hours you put in. But most importantly, when first booting up the game be mentally prepared to be killed. A LOT! No other shooter has prepared you on how to approach battles with hundreds of enemies that may come at you from literally any angle. The learning curve can be eased by joining a squad to cover your back, choosing a support class like the Medic as you take in the fight away from the frontline, choosing less populated fights, asking for help (people will be more than welcome to help you out).
Balancing a 3-way war, on foot, ground and air with unlimited number of vehicles, weapon combinations and terrain of each conflict is an endless endeavour. Especially as game evolves and new weapons / vehicles are added. Right now game feels more balanced than ever (and another big balance pass on LMG's is scheaduled).
In Planetside 2, the things you can buy with real money are XP boosters, weapons and cosmetics.
XP boosters provide more in-game currency to buy/upgrade weapons/abilities faster. All weapons in the game are balanced to be sidegrades of each other, there is no "best gun in the game". Most expensive (not necessarily "best") weapons (1000 Cert Points) can be bought after about ~12 hours of average-skill gameplay with no boosts, although in most cases, default weapons are the veterans' favourite weapons anyway. Cosmetics are, as to be expected in a F2P game, aplenty and the library contains huge user-generated items inside.
I'd call it Battlefield on steroids, but that would be a disservice to it's technical achievement, really. 8*8 km "Continents" (what any other game would call maps..) with 3 factions, battling on foot, ground and air. Combined arms and stragtegy win fights here, as they should. Still, an experienced player can singlehandedly surgically change the outcome of a 96+ vs 96+ battle (there's many of them going on at a time, too).
This is the best MMOFPS ever, through sheer scale, technical achievement and accessibility. Also it's free. If you are into FPS, tanks, flying or strategy, you ought to give this a try. Just make sure you join an active squad and ask for help along the way (or on reddit, community is as friendly as it gets).