Pleasant Surprises:
Splawn QR & Nitro models - To my ears, sounds just like the real ones. A lot of people dislike them because they expect something more Marshally, but I always found them much more suited to heavy styles than anything vintage.
Fuchs ODS - I've never played the real amp, and expected something Dumble-esque, but they have tons more midrange and gain. With a boost in front of them, I get insanely amazing high-gain lead tones out of it.
Let Downs:
Diezel Herbert - Friend has an early pre-production prototype Herbert that I've spent a ton of time with. It's an awesome and incredibly unique sounding amp. Because his isn't a finished model, maybe it sounds different than the production versions, I don't know, but the Herbert model in the Axe just isn't the same. And they didn't model the clean, which is also crazy unique in a funky, hi-fi kind of way.
Mesa Lonestar - This is one of my all-time favorite amps in person. The Axe patches sound decent, but I can't coax tones out of it like I can with the real deal. In the Axe, I'll pretty much always take the Fender models over the Lonestar.