I have an old Android phone, the Samsung Moment so Flash performance on it sucks and battery life on my phone is already crappy without Flash so I don't notice any difference in that aspect and I hardly use Flash anyways so battery impact would be minimal. I only turn it on when I visit sites that I know need Flash. A friend with a Nexus One using Flash on 2.2 Froyo that I've played with for a few hours is obviously an infinitely better experience than on my Moment. He told me he never turns on Flash at all so he wasn't able to tell me if there was a huge battery hit and most reviews that I've read on the Nexus One don't seem to indicate a huge battery hit at all either. It's great to be able to use it to visit sites that need it but watching embedded Flash videos suck on my Moment and my friend's Nexus One running 2.2 was MUCH better with embedded videos as it should be but still not great enough performance to go crazy over. Like said before, most embedded videos were obviously not made with mobile phones in mind. Hopefully with the move to HTML5 that all changes.
See, this is one of those times where I appreciate an Android iPod Touch. If I ever upgrade my iPhone, I might try dual booting though.