+1 -- I'm basically in WiFi h*ll since upgrading. 10.8. Injected all sorts of issues on my network and devices where 10.7 was rock solid. Mid 2009 15" MBP dropping connections on Linksys E4200 at 5GHz. If I switch to 2.4 GHz, no issues. At home I have an Airport Extreme running 2.4 and 5Ghz. I am experiencing drops on 5GHz there as well. I upgraded to 10.8 instead of a clean install, but from reading the forums that doesn't work either. I have a Mac Mini that is wired and no problems.
A couple of other interesting pheneomenons since upgrading;
new model Airport Express:
I bought one the same day as upgrading to 10.8 and immediately encountered issues setting up the AP to extend the Airport Extreme network on 5GHz (kept returning "encountered unkown errror" when writing the configuraiton to the AP). I had to use the old 5.6 Airport Utility to get it to save and 2.4GHz to keep devices from losing their connections. Other weird thing is the 'Back to My Mac' icon on the Airport Express stays yellow where the Airport Extreme turns green.
new model Apple TV 3:
Flawless prior to upgrade day. Now, no longer works on either frequency. Started with dropping Home Sharing connections, then progressed to losing WiFi altogether. It won't even run on the ISP provided router at this point. If I do a reset or a full restore in iTunes, it works for about 5 minutes and then drops the connection and resets itself to the main screen with only a 'Settings' and 'Computers' icon.
I know, possilby conincidences and unrelated but curious issues for sure. I've pulled out each piece and tried to see where the problem returns but no luck. I have recommended and deployed a lot of Apple devices over the years and this is the first time I've ever had problems. One family member seems to be having a similar issue with a new Apple TV3, Airport Extreme and iMac on 10.8 combination. A little embarrassed for having made those recommendations now. The only thing that solves it for me is rolling back to 10.7