I've had a Mark IV for a couple years now, and have been really impressed by the overall flexibility (I generally use a widebody combo with Custom 90 speaker and a Thiele
extension cab with EV speaker).
I've used mine for gigging and recording in several bands. One band is a traditional big band--we play mostly standards, 30's/40's swing, and Latin tunes. The other band is a hard rock/funk hybrid that gets a little progressive at times (
http://www.naughtybutterspoon.com/). Plus, I've done a lot of independent recording, mostly progressive rock and guitar instrumentals. The Mark IV has worked great for all of them. I think the Mark IV's tone is nicely middle of the road--not too vintage, not too modern.
Another amp you may want to check out is the
Mesa Nomad. Another flexible 3-channel amp (though not quite as flexible as the Mark IV), I think the Nomad's channel 2 is more vintage than the Mark and channel 3 is more modern than the Mark.
--B