Alright, so I finally finished the campaign a few minutes ago, here are my thoughts:
I liked the brevity. I normally don't have the drive to finish single-player games nowadays (screw you Far Cry 2), so being able to wrap something up in 9 hours is nice. I could have even done with a shorter, tighter Marine campaign and a little extra for the Predator and Alien.
The Marine campaign just wasn't fun for me. For every genuinely scary moment, there were three "scare trumpets" that signaled absolutely nothing. For every interesting audio diary, there was some horribly delivered comm chatter from Tequila. It just didn't have much polish, and really felt like it would have needed a couple month more work. Combat Androids also suck, and aren't interesting to fight in any way. They just one-shot you with a shotgun, ignore cover and run at you, and are huge sponges for bullets. Really, really lame enemies.
The Alien campaign was fun. I loved the mobility, and trickery you can do as this species. Breaking lights, hissing, and setting up an ambush is brilliant fun. Not much else to say, other than I wish it was longer, and it had more large indoor environment levels (less cramped cabins).
I finished the Predator campaign last, because I had a hard time getting into the controls at first. Took a lot of changes, but I finally got everything smoothed out, and the campaign ended up being my favorite of the three. I wish they let you acquire the weapons sooner, because it would have made for some really fun game play to have the disc in the more open levels at the beginning. The melee combat was fun once I stopped button mashing, and it's just damn fun to feel like you have such a powerful arsenal at your disposal. Even the narrative was kind of fun, which I wasn't expecting from this game at all. I have one problem, being that the cloak isn't as effective as I think it should be (if they knew I was behind a pillar even at 15 meters they'd be able to spot me).
The graphics/sound design was mostly great. I loved hearing the motion sensor as an Alien, and Marine. The sound you get from piercing an Alien with pulse rounds is so meaty, and so satisfying. Predator voice/laugh/breathing was great, and I loved catching it during the Marine parts. Music was generic, and I wish they had something catchy for the menus/lobbies. And again, the scare trumpet is just fucking useless. And screw Tequila as well, she ruined the Marine campaign. On the graphics end, PC gamers know that the game is easy on the eyes. My only complaint about the looks, is that they should probably look at improving the walking/running animations for the bipeds.
I think the low-mid 70s scores are on the ball when it comes to the campaign. It's no honour student, but I can tell Rebellion tried it's best, and they excelled in a few areas.