From an actual person, not a Youtuber who got one for free.
TL;DR - Sounds pretty good, just not $600 good.
The Atomic Amplifire is a new guitar amp modeler that aims to be that middle ground device, between consumer/hobbyist pieces like the POD HD series, Boss GT, and various other $100-$500 modelers, and then higher end devices like the Fractal Audio Axe-FX II and Kemper Profiler (the latter isn't technically a modeler; I know. But it competes with the others nevertheless).
Onboard are a handful of amp models, accompanied by their respective "matched cabinet", along with a very sparse but effective set of delay, overdrive, reverb, and modulation effects.
What sets the Amplifire apart from the POD HD's of the world is that it can load user created or third party IR's, or impulse responses. An impulse response is the resulting wave file from an audio pulse fired out from a computer, through a clean power amp, into a given speaker cabinet, miked with a favorite microphone, then back into the computer, to create a capture of that exact configuration. This feature alone makes it worthwhile in my opinion, over some of the other modelers in its range.
The amp models in general are quite good; the Fender twin model is amazing, probably one of the best cleans I've heard from a modeler, and the various gain tones are great too, with the exception of the Mesa Rectifier modeler, which had a really odd midrange honk I couldn't seem to dial out. There aren't a ton of options here; a couple Marshalls, a couple Fenders, Vox, Mesa, a Cornford, and even a 5150. Atomic promises more amps, as well as more overall improvements to the device, via free firmware updates. This is a very good thing.
So, the inevitable question: how does it compare with the others? Well, unfortunately I can only compare it to the POD HD, a distant memory of an AxeFX Ultra, S-Gear (software), and BIAS (software). I don't yet have access to the rest.
Compared to the POD HD, I would say the amp modeling itself is marginally more refined than the POD is, and removes the fizz often associated with that device. However what makes the Amplifire a considerable improvement over the POD HD is just the simple fact that you can load Redwirez, Ownhammer, or even your own IR's onto the device for use. While the stock IR's are decent, the Redwirez I used still definitely sounded better.
Compared to the Ultra, I'd say overall modeling quality was on par, but the Ultra has FAR more flexibility, including more than one amp model at a time. The Amplifire is just a sole amp model each patch, and the effects of the Ultra are far more vast. Then again, even used the Ultra is generally more expensive.
Now comes the crux of my situation: comparing the Amplifire to S-Gear ($129), and BIAS ($20), two software-based modelers that reside on Mac/PC and iOS/Mac/PC, respectively.
Obviously the fact that the Amplifire is a hardware device bears its own benefits, particularly for both live use, as well as offloading power from a computer. However, for the reason I was buying it for (recording), that advantage is more or less irrelevant.
Comparing the actual amp modeling quality, BIAS (when set up as close as possible to the amps in Amplifire) was every bit as good, and in some cases, better. The Rectifier model on BIAS in particular sounded better, more like a real Recto. Having a lot of experience with this particular amp, it's an important indicator for me.
Compared with S-Gear, the Amplifire often fell short. S-Gear is even MORE sparse in terms of amp models; just five. Effects are also few as well, but to me that doesn't matter; this sits on a computer, where I have tons of effects to use. S-Gear also has the ability to use third party IR's right within the application (or plugin), which is a huge thing as well. S-Gear just feels more dynamic, more amp-like, and seems to "breath" more, which is surprising, given that it's a software plugin.
Ultimately, while I liked certain aspects of the Amplifire, it wasn't $600 worth of hardware to me, given my ownership of the other two. Would I recommend it to someone else? Yes, depending on the circumstance. If you absolutely need a hardware device, and don't want to spend $2200 on a FAS Axe-FX, it's a good device. It's very well built, and feels like it would be sturdy under continuous use. If you're using it just for recording though, I think plugins are still a better route. Or even better, a real tube amp, like a Mesa.
Another option is a new or used AxeFX II, which while I've not had first hand experience with, sounds good in nearly every clip I've heard. Additionally, Fractal teased a new device called the AX-8, which appears to be a stripped down, single amp at a time, floorboard modeler. Hopefully that comes in less than $1600.
For me though, I really love amps, and even for recording, modeling is cool but definitely a distant second priority. Plus, anyone who knows me knows I'm cheap, so there's that.