lol, wow, the win8 haters are out in full force today!
Anywho, for the average user, with a single desktop/keys/mouse setup, it is a take it or leave it thing. Simply put, it will not really add much value.
However, for a power user who wants more performance and control over their system, or someone who lives on multiple machines, or someone who is heavy on cloud utilities, or someone who also uses a WP7/8 device, and/or a tablet as well, then the improvements and inter-connectivity available between devices would be more than worth $15
For anyone with a touch screen monitor, it is well worth the upgrade as well, because win7 touch support frankly sucks.
I had Win8 running on 2 netbooks and my desktop at one point when I was playing with it (CP version), and I loved the integration of skydrive, and that my user settings and files followed me between devices. The improvements in RP I tried on only one of the netbooks, and it was a substantial improvement (added ~1Hr of battery life, among other things). The things I am hearing about the better multi-screen support and other great features for people like myself who do not have a standard desktop sounds promising, but I have not seen them in action yet, so I am not exactly sold on them yet.