There are 25 mechs, 22 tanks, and 21 anti-gravity vehicles, 40 weapons (although many are basically more powerful versions of other weapons, but you can't fit every weapon on every vehicle), 4 special systems (smoke, EMP, camo, a sonic cannon), 27 map challenges (where you try to beat several maps and exit through a starport), and 30 story missions which can also be played freelance.
You can play the campaign, trying to beat every map with a predefined selection of vehicles and weapons, or you can knock yourself out in freelance, where you can choose your own vehicles and weapons.
There are many pilots to choose from; these act as the game's difficulty level. Getting through a level with good ol' 'James' is quite the rush.
There's also custom content available if you feel you need more.
The game's addictive and the soundtrack is godlike. Even when you take a break from it, I'm positive you'll always be back for more.
I'm a bit of a fanboy, admittedly, but yeah, in my opinion there's a lot of replayability. I'd give it a 9.