Kotaku has a writer that says it works pretty well, is faster than Windows 7, installs over an existing Win7 very easily, etc. Seems like other than the speed it's a YMMV thing whether it makes sense to purposefully upgrade or not.
http://kotaku.com/5955658/gamers-relax-windows-8-is-fine-for...
Stardock's got a $5 app (mentioned in the post) that gives you the Start button back and allows the PC to boot straight into the Desktop mode, bypassing the Metro UI completely.

And here's a cool-looking free app that lets you do some customization on the Metro UI if you want to use it:
http://kotaku.com/5957495/make-steam-look-prettier-on-window...

I found a (MSFT-branded) link in one of the comments that says you can get it for $15 just by saying you bought a Windows 7 PC after July 2012. I went through the 2-minute fill-out-these-fields process and promptly got the promo code in my email, no validation/verification/proof required. I haven't yet tried to use the promo code to validate that it works because a) I haven't decided if I really want to upgrade yet and b) it cautions you to ONLY execute the purchase from the PC you want to upgrade, and I'm away from home at the moment. But it's a steal for those interested - you've got until February.
http://www.windowsupgradeoffer.com/

JT