Here is what i experienced testing Rev12 a couple days ago, spent about 2 hours testing it.
First off, i noticed it took longer for Coverflow RC3 USB Loader to load. Seemed to take about twice as long as what i was used to it taking. First attempt to load a game from usb just froze when I pressed LOAD button. After that the System Menu froze right after loading. Overall random freeze/crashes were very frequent. I probably had to hold in the power button to reset the machine about every 5 or 10 minutes, maybe more often than that.
I spent about an hour trying different combinations of Instruments and configuration of Rock Band 2. The good news is I was able to get USB microphone to be recognized and work about 50% of the time I booted the game. The bad news is the USB guitar and drums were never recognized. The even badder news is the network would not connect in game, which would cause the game to either freeze or simply not allow most of the game modes to work. As soon as I reinstalled Rev10 network resumed working in-game.
I haven't heard from anybody else who tested it but I would expect them to have similar experiences. I would have to say based on my experience from a build a couple days ago that it still has some issues to be ironed out. The stability was bad enough that after testing I immediately reinstalled Rev10.