It's a terrific puzzle game with terrific mechanics, and it keeps you interested by rewarding you with funny dialogue every time you beat a puzzle.
I have three criticisms:
(1.) The middle section of the game relies too much on exploration and observation instead of puzzle-solving.
(2.) The philosophy of the game seems to be "only make walls portal-able if they are part of the intended solution". Unfortunately, this sometimes has the effect of making the solution too obvious.
(3.) The single-player puzzles never get very complicated. By the time you beat the single-player game, you feel like you've just been playing the tutorial the whole time. The co-op puzzles are where the real meat of the game is.