I use one as a backup to my Helix. The core tones are very good, effects are ok, drives not so good (although TS is decent), but for live use the 3 button approach is pretty limiting. I need more, so have ended up building it into a board with more effects and boost options, and it has worked out costing more to get what I need than i thought. In retrospect I'd have been better off with a GT100, saved a fortune, but for a backup rig would have been perfectly adequate.
I've also found it to be less intuitive than the Helix to get tones I prefer, and I've spent a lot of time tweaking to get what I like - I still think it can sound a bit thin in a live mix, but I'm sure that is user error. Given the choice when I get in at night I almost always switch on the Helix, and i have found it hard to be inspired to spend the time with the Amplifire that it deserves.
Hopefully this doesn't come across as overly negative - I've actually used the Atomic on a few gigs with a new band I joined, just to force myself to spend some time with it, and got compliments on the tone, and I'm growing to like it more - if you play in a classic rock band and just need a small handful of core sounds this will likely do a very good job.