I too picked this up during the $25 deal and I have to say it was a solid game. I do think it's worth $25-30; as other have remarked, the game is on the short-end, but I prefer games that keep what is necessary and cut out the unneeded fat. There's hardly any "padding" on this game. The story moves along at a constant pace, and rarely are you left wondering why you're being put to task to do something.
The combat is a bit repetitive but that doesn't mean it's entirely lacking depth. You can unlock a few moves by collecting the glowing orbs through Trip's shop. That said, you could probably get away with just button-mashing, but not always. I liked the way NT added the two different weapons to Monkey's staff, though I wish they could have done more with it.
That's one thing you'll probably feel in this game (especially since the ending begs for an epilogue), and that is how much more the game could have been if NT had more funds behind it. I think the story is excellent -- for a videogame story. Despite the great animation and awesome emotional expressions in its characters, I felt the characters could have used some more development, but Enslaved does a much, much better job than most games out there at getting one to relate to the plights of its protagonists.
I would love to see more games like Enslaved; gameplay certainly takes second fiddle to the story here, but it successfully keeps you engaged to finish, and that's saying something.
Definitely worth a play-through for every gamer. It's a shining example of what can be done with the medium, and it's easily worth it IMHO at $30.
So, unless you are into video game "stories", this game is not worth $25, let alone $30. Having said that, other people seem to love it for the story, so YMMV.
