its autofocus is meant to be about the best there is, although I've never used one. I was tempted by a second hand mkII (not mkIIN) when I was buying my 40D but eventually settled on the 40D.
I'm sure it'll be a pleasure to use. The only negative point would seem to be its 8MP compared to 15 on the 50D, so images will be less 'croppable'. I'll be interested to hear how it compares to your 30D.
Matt