yogsloth: My kids (age 6 and 7) love it. It's pretty neat. Very hard, though. We're totally stuck because the next achievements we're supposed to get are all impossible!
The full version has changing maps every day, and if you want to log in to their server (not required), you can download all kinds of maps.
My son is 5 and he's super keen, it seems. He sits in my lap and leans with the racer. Impacts are quite jarring for both of us.
For me, the full version control response feels much better. The world elements are more varied, and less random. Plus, new levels and items are much more accessible. Maybe there are more than in the alpha, or maybe the better controls allow me to level up more easily.
Plus, ramps, windmills, elevated pathways.
Coolness.