Although not really mid-gain I usually like to point out System of a Down's Toxicity. The whole album is basically a showcase of the many of the different sounds a tweeker can pull out of a Dual Rectifier......there is some Marshall work on that album too, but all the main "out front" tones are Mesa.
Some other bands to check out (I'm going for an assortment of styles, so don't flame me) would be Dream Theater, Soundgarden (Superunknown), Alice in Chains (Dirt), Crowbar (Sonic Excess), Linkin Park, Limp Bizkit, Korn, P.O.D., Blink 182, Godsmack, Disturbed, Tool, Stabbing Westward, Foo Fighters, Mudvayne, Nickleback, MXPX, KingsX, Incubus.
Bold = bands that I think might have mid-gain tones going on (but I could be wrong).
They cleans are good. They can be hard to dial in as the clean channel has enough gain on tap to bury alot of other amps "lead" channels, and hot pickups exacerbate the issue. When you dial them in they're quite good, but they're quite modern/cold sounding and alot "stiffer" feeling than traditional Fender or Boogie (Mark II, III, & IV) cleans. I've noticed that alot of bands that rely heavily on cleans tend to add a separate amp....both Korn and Limp Bizkit for example were using Diezels for awhile.