My take was that it gets much better, the demo is the worst part of the game in my book.
Having said that, after buying a WiiU this year and playing some of the platformers I missed last year, it's really hard to look back at it quite as positively. I enjoyed it for what it was, but for something some of us talked about reminding them a bit of Nintendo games, it just ends up showing how divorced from Nintendo a lot of us must have been at the time. It's the best actual complete game Media Molecule has made, for what that's worth.
I hate to sound negative, the game was a lot of fun for me as evidenced by it being in my top 5 last year, and I happily own a physical copy. It's more of a PS1 action adventure game, and there were plenty of decent ones of those it could rightly be compared to, it's just pretty lightweight in the gameplay department in retrospect, and the difficulty is essentially nonexistant.
Completely subjective here, but I certainly think $18 is a fair price, but I wouldn't pay more.