It's fun enough, but somewhat limiting. You are definitely firmly attached to a rail that leads you inevitably from one set-piece to the next, and they get larger and more explod-y. And the rail is often quite obtrusive. On the plus side, the world is visually interesting - coming off Fallout 3, where the postapocalypse is unrelentingly grey, Enslaved is at least a very colorful world. On the minus side, I think the developers worried you couldn't see the ledges and pipes that you need to run around on, so they made them glow a bit. It breaks the realism, and it shows you your path laid out like fate in front of you.
Andy Serkis voices the main character, and was also involved in the motion capture and voice acting process. I think it shows. The acting is top-notch. The story has gotten a lot of attention, but I don't think it's really any more than adequate, though the epilogue has a hint of something better, though it's chucked in at the last minute without much relevance to the rest of the game.
The actual game play was fun enough, but I think they could have done more with the interplay between your character and your captor/companion. In some of the early chapters, there's a certain amount of teamwork and strategy (even if your companion's contribution is to do exactly what you tell her to do). I thought it was building up to something where you'd have to be simultaneously smashing robots and giving her orders, or smashing robots here, while making sure to protect her over there. I was anticipating having to send my brain into full-on Ender Wiggin mode. Instead, the interplay between the two characters actually decreases, and mainly you just fight bigger and badder mechs, solo. Boo.
Also, it seems like it was over pretty fast. The main story of Fallout 3 took longer to complete, and there was still lots more fun to be had roaming the landscape.
But I did spot colleency in the credits, which made me go 'Yay!'