I just finished playing this game not too long ago. My mind has been officially blown. Everything about this game is HUGE, from the levels, to the baddies, and the guns.... oh yes... the guns. At the heart of this gem is a simple, "run and gun" shooter, but what sets it apart is the beauty and care that went into the wide open levels (unlike Doom which is spent mostly in corridors) and the RIDICULOUS amounts of enemies you must face. For throwing literally hundreds of enemies at you ALL AT ONCE, it maintains a smooth frame rate and the pace doesn't lag. The guns continually get bigger and bigger and each one can deal with enemies in a unique way. One thing that surprised me about this game is that if you pay attention to the monsters, they all have patterns which, when figured out, allows you to kill them much more efficiently with certain weapons. There are plenty of laugh-out-loud moments (particularly Sam's wisecracks) but these are typically followed by "OH MY GOD" moments, so you quit laughing pretty quickly. There are very few puzzles to solve, which, for those of us who are used to the frustration of scouring level after level in Duke Nukem 3D or Doom, is a serious blessing.
I could go on and on all day about how great this game is, but you need to play it for yourself and see first-hand how awesome and fun it is. $10 for a game of this magnitude is a trifle. Get it now!