I'm fairly close to the conclusion of this game (about halfway through act 4 of 5), so, as you can guess, it's not the longest game in the world, but there seems to be enough options there to make you want to complete it a couple more times. Not to mention the online play, which I look forward to. Was playing 2 player split-screen earlier, which was great fun.
Personally, I like the fact your team-mates are quite vulnerable. Baird sometimes pisses me off by jumping over walls in front of me, and getting in my line of fire, and often making me think he's an enemy, but they've saved my skin a couple of times, so they're not too bad.
The difficulty is about right. It's tougher than most shooting games, but without being overly frustrating.
For those that might have read spoilers, walkthroughs etc, don't confirm this, as I don't want to know, but i'd put my bottom dollar on 'The Cole Train' being a turncoat near the end.