It's awesome! Go get it.
The game is 10 chapters, I've been playing for around 4-5 hours and I just got to chapter 5. The game is hilarious at times, and just plain fun. It takes the same kind of puzzle solving from the first, and adds a whole bunch of fun stuff to it. Light bridges, bouncy gel, lens cubes to change the directions of lasers, catapults, and I think a couple other things that I have yet to see. The puzzle design is as great, and most of the time exceeds the designs in the first. Characters are great, voice acting is great, and the story is interesting. It just does a lot to flesh out the whole aperture science universe.
Thats my short review. I love it so far, and I have yet to touch the co-op. Have the day off tomorrow, gonna Portal it up!
I bought an Aperture Science mug and a t-shirt from valve...that I will be drinking coffee out of in the morning whilst wearing my sweet shirt.