I think it's a pretty good way to distrubute the game, but you're able to buy the full retail copy for much cheaper.  Peter Molyneux has said he doesn't like demo's, so this is basically his alternative, it lets you play around with the first few parts of the game with no restrictions, and if you're hesitant about buying the game, I'd definitely recommend checking it out first.
 
Achievements will work, but only when you buy the other parts, they're disabled from this one because it's free.