For well-behaved applications like web browsing and email, this works very well. I have never tested this with voice, but I don't expect it to work well or even at all during a call.
802.1x clients perform the best and they take between 7 and 30 seconds to fail over, depending.
With regards to the PMK, it is not much different than having a client roam to another access point, so it works fairly well. If you bridge clients to the same VLAN, the ARP table does not have to change, etc.