A third person view mech shooter variation on tower defense genre, dipped in signature Double Fine humor sauce. If that sounds appealing to you, think no more, get it. It does its job very well!
You're a pilot of so-called mobile trench in a campaign against Monovision ravaging the world after mysterious Broadcast. The tongue-in-cheek campaign consists of missions tasking you with defense of a strategic point(s) from waves of enemies spawning at concrete locations on map and converging on their target. You fight partly yourself directly, dealing out punishment from guns mounted on your mechanized trench, and partly you summon turrets for help, paying for them with scrap gathered from fallen enemies.
There's a big selection of guns at your disposal - machine guns for relatively small but sustained damage, with flak variety focusing on airborne enemies, and a wide selection of burst damage weapons varying in range, area of effect, and reload times (shotguns, sniper rifles, grenade launchers, even artillery cannons). There's also a unique type of weapon - broadcasters, with huge, continuous damage, but very short range and long time to replenish (sort of like EMP version of flamethrower, I guess).
And all these guns also come in a standalone, turret version, automatically tageting enemies in their range, except broadcaster type of turret is not for damage but rather a support one, slowing enemies down, and there's a type of turret that lays mines rather than shoots at enemies. Other support turrets include magnets that automatically gather scrap for you or can repair your trench.
(by the way, normally your mechanized trench CAN'T be blown up during a mission, you're just disabled for a short time and only lose when defended target is blown up, however stages of campaign end with boss fights where you yourself can be considered the target and then repair turret sure comes handy - or pretty much required)
There's a gotcha however - a harsh limit on what you can actually bring with you to a single mission, and mixing and matching a loadout before launch is big part of your success. Generally you can either go with a chassis that can dish out huge damage itself with lots of gun mounts, but very poor selection of turrets to summon, or a chassis that can deploy powerful turrets but has very limited space for guns on itself (there's also a middle ground, having an average number of gun mounts and limited only to light offensive turrets). These also differ in speed and armor values, and come with a choice of legs that give you some special move, for example let you sprint very fast in straight line, or give option for faster reload times if you root yourself in place. There's also a smashing variety of legs that complements your damage potential.
The selection of guns is ever expanding in their power and sometimes additional effects (like more scrap, or magnetic effect on grenades - very useful perk), you have this feeling of constant progress in power as you level up your pilot, very well realized indeed (but of course mission difficulty also ramps up to match it).
With rumors on GFWL closing down soon-ish and Double Fine not planning to port this game over to a different platform, it may be highest time to get this one, too, before it becomes unplayable (though it's not clear if it won't work at all or just the co-op version? btw yes there's co-op)