Well, I loved the game myself.
Some parts to the game had my eyes glued to the screen because it looked so amazing. Like when you are part of a squad running towards an enemy base with gunfire going off all around you and jets zooming overhead to bomb their target. The tank level was quite hectic and a nice change of gameplay, I liked it a lot.
Everyone seems to be saying that the Alien hive, zero gravity part, was bad but I thought it was pretty good and a nice change from all the jungle fighting before it. After the hive, when the snow is everywhere, I also found brilliant. It feels like such a different place with a completely different atmosphere, the way you start to freeze up when you stand still for too long and your reflex sight is so badly frozen over that it is useless is great!
Fighting the Aliens made a refreshing change to gameplay, forcing you to change tactics from ones you have become so used to. Get up in their faces with the shotty! They weren't difficult but if you let them swarm you, you were as good as dead.
The last level, on the carrier, was fun. It was quite slow paced compared to the rest of the game before but when the Aliens started to attack the atmosphere was tense, not knowing how many, if any, Aliens are going to be behind the next door you punch open.
The fights on the deck were quite good, the big walking thing takes one hell of a pasting and I was getting worried that I was going to run out of ammo before it died but fortunately I found a much needed minigun. The last boss was a slight let down because it just sat there and pretty much let you destroy it, I did die more than any other part of the game due to all those flying aliens that come out(Tip: Use the Gauss rifle to take them out instead of the rocket launcher).
The ending annoyed me because I was pumped and ready to go kick lots more Alien arse but then the credits popped up. I thought it was a joke and waited, staring at the screen for a while in the hope that gameplay would commence, but it didn't
Overall though, I really liked it. It had so many memorable moments and times when I sat there with my mouth open in amazement that I couldn't not like it.
*EDIT* Oh yeah, the VTOL part was good fun, too. DON'T GO NEAR THE CYCLONES!