There are many MMORPGs out there at the moment... but this isn't one of them. Planetside offers the oppurtunity to take place in what amounts to a MMOFPS.
Three empires slogging it out for control of 10 continents sometimes with hundreds of people competing for control of bases in a variety of armours and vehicles.
The game DOES have a slight roleplaying element in that as you gather more experience, you gain levels and hence can get access to implants and more certifications that allow access to the games difference weapons, armour, vehicles and abilities (engineer, medic, hacker). As well as these battle ranks there are comamnd ranks, that allow you to direct your fellow team mates.
The only game this has any real similarity to in my experience is Tribes but this is Tribes to the nth degree. Each continent can take minutes to fly across in the fastest vehicle and sometimes when there are dozens of vehicles and troops running at each other, this game can have a truly epic feel to it.
Naturally though, it can't all be epic confrontations with 100+ people. Sometimes you'll be stuck in a base waiting for its capture or having to drive around an ANT to repower bases.
The teamwork can also be a bit hit and miss. Sometimes you might as well be working on your own. This is naturally combated by the use of Outfits, the Planetside equivalent of Guilds. They generally tend to be far more organised and offer a better teamplaying experience.
With all the various aspects of the game, there is always a task for even the least capable FPS player to accomplish and as you're all fighting on the same side there can be a sense of camaraderie in the final push for a base or the last ditch defence of a continent.
Where the game is weakest though is the absolutely horrible HORRIBLE stability. Playing 3-4 hours a day for a month, it wasn't at all unusual for the game to crash once, twice or even three times a day. This can be a real nightmare if you're driving a vehicle or doing something important for the team and yet the technical support offered is pitiful... for a game that has been out as long as this has, the game is shamefully bug ridden and even managed to blue screen me a few times. Sony Online seem content to deny the problem though.
This game will NOT offer you the kind of instant gratification you'll get in a DM or the long term goals that an MMORPG can. You're playing more for the experience of playing rather than any particular notion of advancement. These experiences are totally variable though and at times, the game can become frustrating as the necessity to run out, die, respawn, run out, die etc. gets in the way of fun.
And like all MMOs it can become a big time sink, though this is less true than it is for other games. It DOES offer a considerable number of different approaches and it is hard to beat the rush of raiding an enemy base. This game is really for someone that likes the idea of a giant persistent game of Tribes.