The goal is simple: roll your cube from the start to the end of the stage, collecting any glowy things, and trying not to fall off. You're ranked based on the number of glowy things picked up, and the number of times you plummet to your doom.
The joy of the game comes from the tricky things you've got to do in order to reach the goal. There are the standard platform dangers - moving bits, touch-and-drop panels, springs and so on, but then the game gets clever.
When your cube is rolling, the edge in contact with the wall or floor is locked in place. This allows you to, for example, roll up and hover by tapping against gravity. As long as you're stuck to the wall or a platform edge like this, you accumulate 'edge time' which counts against your stage time.
This same trick allows you to stick to the side of a moving platform, or hover against a wall with no floor, waiting for the floor to come back. The thrill of trying to balance against the side of a moving platform, where nudging your cube the tiniest bit too much or too little will drop you into deep space is palpable.
The on-screen d-pad works as well as you'd hope, though you can also use a swipe-and-hold control which I don't really like. There's a ton of levels, chiptunes for a soundtrack (download here!) and gorgeous minimalist graphics.
If it's not the best iphone game, it's pretty close.