I like the atmosphere of the game. The lighting and the art matches well. Overall nice gameplay, and the little puzzles are great. The difficulty level increase is a little bit skewed in the beginning, but after that it increases nicely. I stopped at the elevator level, as I couldn't get past the second elevator without getting hit by the spikes.
One tip: games like these rely on the speed of being able to start over when you die. The character starting in big mode and then shrinking down to gnome takes away a lot of this speed. Being able to immediately continue walking when you die (ala Super Meatboy) would be a huge improvement, imo!