I saw lots of requests for the 2c on ideascale, some for Mark V, but exactly zero for the Mark III, though I might have missed them, search there isn't awesome. I like the other AT Mesas too (except the TA, which I expected to love and really don't), but the Mark III is my fave.
Just to be clear, I'm NOT after super high gain. What's special to me about the AT Mark III is how it behaves when it's overdriven a little but not a lot. Specifically, as I said, I like running the master hot and channel gain low. Looking for some fatness and sort-of-compression-but-not-really, might be sag, and I love how it digs in when I pick harder. I often have several different overdrive stomps set up in a patch like that, so as saved it's clean but fat, then I can kick in one of the higher gain stomps for a solo if I want, but it's still nowhere near metal territory. Oddly enough, one common overdrive I use is just the 7-band graphic with some gain and tailored EQ. On the Mesas especially, that can sound just awesome to me. I've also found uses for most of the overdrives and some of the fuzzes, I just don't jack em up that far usually.
One thing that makes the AT Mesas work so well for me is that built-in EQ post the preamp. (Not that big a fan of the rack EQs in AT, though the parametric is useful.) I often kill low end before the amp to prevent flatulence, then fatten and brighten it up after w the Mesa EQ. Clearly, adding EQ blocks anywhere I want isn't a problem w Helix, so I expect to be quite happy on that level.