Short version: It's great!
Basically, you tilt the iphone/pod to control a sliding, slippery puck of a thing from one end of a long vertical course to the other. It doesn't scroll horizontally at all, but there's a lot of sideways tilting as you zig and zag down a course, avoiding lasers and spikes and precipitous drops. The emphasis is on speed, but the puck's max speed is achieved almost instantly, and it's not that fast. The player is therefore tasked with efficiency, and it's soon second nature to grab bonuses with the utmost restraint. Get the orb and get back on track before the sliding pusher comes back and interrupts your flow.
It has global rankings and facebook updates, but I keep coming back to it for the gorgeous visuals, smooth animation, and challenge.