I upgraded by MacBook Pro to OSX 10.8 Mountain Lion. So far, so good.
EIgenD performs fine. As does the driver for my audio interface (M-AUDIO Fast Track Pro).
I've tested several AU/VSTs, including all from AAS, Tone2's Saurus, u-he's Diva and Zebra2, and Camel Alchemy -- all seem to perform just fine.
I've not done full tests on the DAWs I use, but all are reported to run fine (from their respective websites). They include REAPER, Reason and Studio One.
I'll probably wait a bit longer to upgrade my iMac, waiting until there is official notice from the software publishers.
Randy