I finally have most of my managements (we've acquired several companies, and haven't had time to merge the managements) upgraded to R81.10, and I've been using this feature. Just upgraded a firewall from R80.30 to R81.10 in ten minutes. It's great.
I did hit one weird glitch. My management server and the firewall I was about to upgrade both had CPUSE 2205. I told the management to start the upgrade on the firewall, and it complained that I had to update CPUSE first. Turns out the firewall had somehow found out about CPUSE 2208, but the management server didn't know about it yet. The firewall can't connect out to the Internet (so I'm really not sure how it found out about 2208), and running 'installer agent update' on the management told me there was no update. Eventually, I downloaded 2208 from the Support Center, uploaded it to the management, copied it to the firewall via cprid_util putfile, installed it on both, then the upgrade worked.
This definitely wasn't a big problem, but it was surprising given how little manual intervention the tool takes otherwise. It would be nice if the firewall's knowledge of a newer version didn't prevent the management from working on it.