Its been very stable for me so far. I've had no crashes and aside from a few minor bugs, 10.11 is very nice, even for a PB1.I'im interested in running this. I've got a 2011 MBP (only one of course) and I don't have enough space to run a partition. I've been doing Yosemite beta since it came out. I've trying to delve into the bugs and if PB1 is worth it so far.
thoughts? Yes, I'd have to run on my main machine.
I still wouldn't recommend putting it on your main, and only, drive though. You don't always know which of your apps will have issues or what future PB releases may do to your system.
You'd be better off buying an SSD, putting it into a USB 3 enclosure and running it from there.
But if you do overwrite your good Yosemite installation, be sure you have a good and current backup available in case you have to revert back to it.