I bought a FM3 with the idea that it would be my personal travel rig for when I visited friends in New England and traveled abroad on vacation but I realized that it took up a significant amount of space in my carry-on sized luggage and added more weight than I wanted as well.
What I wanted was something that would fit in the pocket of my gig bag and wouldn't add a lot of weight when I put it on my back thus taking up no space in my suitcase at all.
Even with all of the cons that some felt the Ampli-Firebox Mark II has, the form factor was a big plus for me. Nothing else I was considering was that small and light except the Simplifier Deluxe. I tried one but just didn't bond with it.
Reading the positive reviews and hearing the clips I figured the Atomic tones would be good enough for my purposes; mostly practicing at friends houses late at night or in hotel rooms when traveling abroad. If asked to sit in, like I was when in Colombia, I could put it in front of any amp in their backline and it would be fine for the occasion.
Given my needs for it, the fact that my FM9 will almost certainly sound better is a non-issue. If I can get a good clean sound with good reverb for playing Jazz, and a good lead tone for when I want to rock out, that's all I need. From what I can tell the Ampli-Firebox Mk II has that well covered.