It's pretty fun but the playerbase is still low. You get 1.5 hours of offline mode a day, which is fun if your internet blows like mine does sometimes. I like that you can play it inside of Google Chrome too (though that version is still buggy). And it's F2P.
The gameplay is really cool. It's like Dominion in that you're ALWAYS doing something, and there is sort of a capture-point system. You do have to physically destroy the other player's main base (it's like a Nexus), but having more capture points makes it a heck of a lot easier. You're also constantly building things like in Starcraft, and there's some micro.
The purchaseable in-game-content doesn't break PvP, and it's pretty cost efficient too. You can pay $5 to get a VIP status and then you'll permanently have 10% increased IP gains, among other various features.
My only gripe is that there's no real incentive to destroy opposing Airmechs (they're like champions). The only incentive is that they have to wait 10-20 seconds to respawn. Other than that, the gold gains aren't that high. And there aren't very many types of Airmechs yet, but it's still in Beta, so they'll come in time.