Good game. Good improvements over the first Shadowgrounds.
Much has improved in this game, in comparison to the first. Better voice acting, more purchasable upgrades. And my favorite, you get to play more than one character.
The story starts you off as a marine, and as the game unfolds you switch between a pyro/flamethrower character, and a sniper (rail gun speced). Each character has its own weapon upgrades, and most interestingly, its own "personal upgrades". The upgrades are specific to that one character (such as the sniper, which alows you to slow down time, and rail gun hits are instant kills). This adds just that much more progression, and keeps you wanting to play the game just that much more, to be able to play the different characters, and see how they progress.
They also added physics to the engine, bodies fly around after throwing a grenade, blow up an explosive barrel, and boxes and bodies in the way get tossed around the map. Its quite enjoyable to watch.
I really enjoyed this game. Its pretty short (6-7 hours of gameplay, though 9 episodes of the game), and also easy if played on normal difficulty. I would advise playing through it a second time on a harder difficulty, to make the game that much more enjoyable. At least thats what i did. Give it a try, its worth the $7 pricetag, imo.