Mitchell's rhythm/lead mix was a perfect complement to Hendrix's style, and they always seemed to enjoy playing together, from what I've seen of live clips. Mitchell's work on "Hey Joe" is some of my favorite drumming ever (not that I'm any great connoisseur of drumming, but whatever), and I think he deserves to be mentioned in the same breath with guys like Bonham, Moon, and Baker in terms of guys who defined rock drumming for subsequent generations.
For those who may not know, he was in Black Sabbath, Rainbow and Heaven and Hell. Also, of course, his namesake band Dio. I can't believe he was 67 years old! Peter Steele and now Dio; not a great year for Heavy Metal.