Played the XBox 360 single player all weekend and beat all 3 campaigns on normal difficulty.
Easily a solid 8.5, especially if you are a fan of the franchise or respective series.
The only glitch I had was one time I fell through the map and died, but other than that I had no graphics or audio problems. The checkpoints seemed okay to me, at no point did I feel like I had to unnecessarily replay parts of the game.
The Marine missions are the longest, but nothing you wouldn't expect from the series. Corridor shooting, survival horror. The Smartgun is fun to use, but the shotgun is hands down THE weapon for the marines.
To me, the Alien and Predator campaigns were the most fun to play. The Alien is especially fun once you get the controls down to where you can attack and climb walls at the same time with ease. It's very rewarding to take down a platoon of soldiers one at a time then escape to the safety of a corner on the ceiling to plan your next attack.
The Predator campaign is the best videogame version of controlling a Predator so far. All the cool weapons/gadgets, and the ability to freely target jump to locations (ala X-Men Origins: Wolverine game). Also, the invisibility is unlimited, although there are a few things that cause it to go away, so you can take your time and have fun with your prey. The cannon is power based, but there are plenty of opportunites to replenish your power meter in the game. I found myself only using it occasionally. The spear is the best weapon of the game, but you only get it very late into the game. The Throwing blade is surprisingly effective and rarely gets an instant kill but it satisfies when you do.
Some may complain that it is short even with 3 campaigns, but it seems like there is a lot of replay value and I haven't even cracked the MP yet.