I've had this one for awhile and finally got around to it.
The graphics obviously didn't age well, but it's a decent game.
The storyline goes that you're a former Marine (and now tougher, let Gordon explain that one) who is a marshal of the cosmos. Your job gets you involved in the hunt for some alien artifacts that could be the most dangerous weapon in the universe.
The game starts off sort of slow, but tough. The early levels are uninspired. Truth be told, the whole game design is old (obviously), but the storyline and variety of missions do a pretty good job of keeping the later levels interesting. Most levels are also on the short side which helps keep the pace up.
Interestingly, IMHO the toughest bad guys in the game were the initial ones. I think it was mainly due to the weapons available at that point and their lighting quick movement. Throughout the rest of the game the enemies are normal humans or Skaarj (remember them from the first game?). Of course, it could've also just been a learning curve in learning the movements and weapons. In later levels the weapons change and it gets a lot easier to bring down the thunder without scrambling little beasties everywhere.
The storyline as I said is pretty good. The bit players in your little world are all full of enough detail that you'll be interested just enough to seek them out between missions to hear what's going on. The ending, in particular, is satisfying and leaves you wanting more as any good ending should.
The weapons are an odd mix and one of my only criticisms of the game. The majority are gimmicky, fill only scattered situational moments, and take forever to load. The ones you'll really feel comfortable with are shockingly the shotgun, pistol, flamethrower, machine gun, and rocket launcher. Your get a grenade launcher whose EMP rounds will be pretty invaluable too.
Overall, if you find this game for $5-10 bucks, pick it up. It's very much worth it in that price range. Nothing will blow your doors off, but you'll be entertained, and that's the point, right?
6 out of 10 (although I assume at the time of its release I might've given it a higher score.)