I have one BB Plus (which is modeled in the Axe 2) in front of my Axe. It is a dual pedal, but since I keep it rack mounted, I just keep the overdrive side engaged, and I bring it in and out with an audio loop switcher. I hope to send it out to Xotic soon to be modded so I can also use the Distortion side and have the audio of each half of the pedal remain separated and going to it's own audio loop in my GCX.
I prefer stacked effects - such as overdrives and delays, as opposed to one effect with the mix turned up louder or the gain turned up higher. So sometimes I will create an effects chain with only one drive block or perhaps 2 drive blocks, and there will be certain situations where I will literally have both drive blocks and my outboard BB plus engaged! Each stage is very low gain by itself.
Believe it or not, scenes and my discovery of using 2 amps in parallel, decreased the frequency where I've needed to stack so many overdrives, since I can do more gain with the amp model itself, and keep separate amp gains at the ready, for recall with a scene.
My BB Plus costs about $225 by itself. All it does is gain. And it does it very well. Is the Axe FX good enough? YES!!! But I already owned this BB Plus, wanted one for a long time, and finally got it, only about 18 months before I got the Axe 2. The BB plus is a religious experience if you put it before a tube preamp, and it does an awesome job before the Axe 2 also. I will probably never get rid of it. In fact, when I think to myself of other areas I might want to improve my rig, it would be to add different overdrives. Some of them might or might not be modeled inside the Axe2, but the real ones sound fuller to me, and again, if I do handle gain from outside the box, I don't need to use a drive block, and I can use the CPU for something else.