I've got a similar thread about this posted at Slickdeals. After many conversations with other members, it appears that this is indeed an upgrade version. I have installed it from scratch on a bare drive so the term "upgrade" isn't all that accurate from an installation perspective. I was never prompted for an installation disk nor did the installation program do any sort of search for a pre-existing Windows installation to upgrade. Based on my past experience with Windows 7, Microsoft basically sells the same software at two very different price points. The fact is, you can install an "upgrade" as a new install and get the same benefits as the full version, but at a fraction of the cost.
Now for the caveat. I have not connected the PC to the internet or activated the software. I just wanted to install it on a spare PC and test out the interface before I commit one of my PCs to a Win 8 setup. I have no idea what will happen when and if I make the connection to the web with regards to the status of the installation. I really don't expect anything to happen other than getting the software activated normally, but who knows?
For an OS that's supposed to run faster, I find that it's extremely sluggish and slow. I have to wait a bit before it starts accepting commands, such as moving the mouse to the corner to display the menu icons. I installed it on an older SATA drive and not an SSD so I plan on doing that next to speed things up. Once you get past the weird new UI, the underlying structure appears to be pretty much the same as Win 7. I really haven't looked into what changes they made to the OS other than the obvious so I'm not sure what I'm looking at yet. I'm mostly scratching my head and asking myself "WTF did they do that for?" I only bought it because it looked like a good deal at $40, plus I was more than a bit curious. I have a feeling I'm just going to think that it's $40 I'll never see again.
FWIW, I got my license key for the Media Center upgrade. Big friggin' whoop.
I can't see using this OS as a Media Center PC, but I still have to play with it a lot more before I make a final decision. It could start to grow on me if I give it enough time.