Downsides? It's not cheap at £849, and 64Gb storage isn't going to be enough for the MacBook Air to be a primary machine (try storing your movies, photos and music in such a small space). For power users the screen is too small, the processor too slow (editing movies is probably a no-no) and the lack of an optical disc drive could be an issue.
Would I buy one? I think the answer is a definite "Yes, if I didn't already have an iPad". As it is I'll probably wait for the new MacBook Pros (a refresh is due any week now if Apple stick to their normal MBP upgrade cycle) before making a decision.