First my boogie pedigree:
Several Mark III's, several mark IV's, quad preamp, studio 22, rectoverb, nomad (disclaimer: nomad for three hours before returning), lots of cabinets (combos, thieles, rectos, trads, halfbacks, 3/4 backs, etc. ad nauseum), triaxis, 20:20, 50:50, probably some more but I can't remember. No mark I or IIC but a good buddy has each. I love the company and their products and I'll always have one around.
Live, I was a die-hard triaxis user for more than 15 years, from the early 90's until.... I got my axe-fx. It's really, really, really that good. The guy who designed it was a triaxis user, and he is the modern-day Tom Scholz, a guitarist who knew what he wanted and built it himself. He didn't just model amps, he modeled the individual components and built the amps inside the thing. It is utterly amazing in terms of the tactile sensation, you can feel the (simulated) power amp sag when you hit a note harder.
I did a review on the fractal forum in which I plugged my triaxis into one channel of my mesa 50:50 and the axe-fx into the other, ran them into identical cabinets, and a/b'd for a few hours. There were a few things I couldn't exactly duplicate, but for most of the triaxis modes, I could get to the point where I absolutely could not tell the difference. Other things were very, very close. When I added my TC effects processor to the triaxis, the effects in the axe smoked it. And of course there are all kinds of fender, marshall, and assorted boutique sounds in the axe-fx that the triaxis won't do.
Special info for those of you who like midi rigs:
1. Seamless preset changes
2. Delay and reverb spillover.
3. Global tap tempos (on or off per patch). So you can tap a delay tempo and it follows you thru patch changes.
4. Programmable harmonizer.
5. Did I mention delay and reverb spillover????
6. Seven-pin midi cable/phantom power jack like the triaxis.
7. Weighs very little.
8. Sounds great direct thru the board.
9. Separate direct and stage rig volume controls, and you can set it up to have cabinet emulation to the board and bypass it to you power amp and stage speakers, sort of like the recording outs on the triaxis.
10. Ungodly good tone, not only mesa tones but all kinds of other tones from Dr. Z to Diezel to you name it.
11. Multiple continuous controller and virtual switch channels, with gazillions of cc-controllable parameters (with no zipper effect).
12. Internal cc wah that actually works.
The "little window" does not drive me nuts at all. The thing is **** easy to navigate. I'll always have one sweet tube amp around, but this thing eats the lunch of everything I've ever owned in terms of having lots of sounds at my feet.
I hate to be such a cheerleader, but you asked.
It's the bomb for anyone who wants a gazillion sounds without pedal dancing and is getting tired of hauling heavy tube gear. Having said that, if you only need a few basic sounds, you should go with a sweet tube amp that does what you need.