Here we have a review from Piaw Na's blog about his experiences upgrading to Windows 8 for only $40:
All in all, for $40, I feel like it's a decent deal. I wouldn't pay full price for the upgrade, especially if you don't have touch on your desktop. I wouldn't go out of my way to do an upgrade (I did, but I was also curious about the new UI), but it's nothing to run away from either. I certainly don't see it as the disaster some pundits have been saying. Mildly recommended."