I'll have to try a performance comparison, but the native HTML 5 video only uses about 10-11% processor power on my MacBook, allowing me to run it on batteries longer. I doubt that even Chrome's integrated flash can provide that sort of performance.

That's true on my Macbook as well, but it's also a lot less stable for some reason. Flash plays perfectly fine, while HTML5 video on YouTube in Chrome will fairly frequently cause weird graphics glitches or freeze a tab. Works perfectly fine in Linux, so not sure what the deal is.

Battery usage is dependent on both cpu and gpu power draw. It's more complicated to determine what the overall difference is between html5 and flash power drain.