I have a late 2011 17" MBP and did a clean install of 10.8. Ever since that time, I am unable to maintain a useable connection to certain access points. At first it seemed like the internet was slow, but eventually pages loaded and I didn't think too much of it. But then I started accessing lots of files on my home server (OS 10.6.8 Server) and the Finder was beachballing with nearly every click. I plugged in an ethernet cable and all the problems went away. Went back to wireless, the beach balls and sluggishness came back. Then I rebooted to my 10.6.8 partition and connected and used the network just fine on the wireless, as I had been for years before on this MacBook and my previous MBP.
After searching through forum posts I saw that some people were blaming this Mountain Lion bug on whatever runs the 5Ghz connections. So I forced my MBP to connect to the 2.4Ghz radio. While I did have one glitch (a video didn't play smooth over the network initially) it has basically been solid ever since. Yesterday I spent the day at a client where I run the network and my 5Ghz problems came back. I would have to power cycle the Airport to get the connection useable again. Both my home network and this client's are running with Ruckus Wireless ZoneFlex 7363 access points. At home I also have an Airport Extreme from a few years ago that I use as a router with wireless disabled. I turned that on and while I didn't really use it for all that long, it seemed to work at 5Ghz--but I didn't really use it that long and need to do more testing.
My exact setup:
1) Clean install of 10.8
2) Late 2011 MBP 17"
3) Ruckus Wireless ZoneFlex 7363 firmware 7.3.x
I hope this is fixed soon or someone discovers a workaround.