I've owned the FV-500 pedal. Solid pedal, but takes a lot of pedal-board real estate. Also, a number of features that I'd never use, using it just as a simple expression pedal.
I'm using two, unswitched ME pedals. One spring-loaded, one friction-block configured. I agree with the comments about spring-load being a very cool aspect to a pedal with the auto-bypass feature of effect blocks within presets on the Axe-Fx. Extremely useful and convenient!
The latency of the auto-bypass/engage is adjustable within the Axe-Fx. I set mine to "fast." Works for me.
Also, my experience has been that connecting the TRS cable from the pedal directly to the Axe-Fx *does* make a difference in improving the responsiveness of the pedal's movements. As opposed to connecting a pedal to my Gordius LG2, and having CC messages sent to the Axe-Fx for interpretation. I don't know if the analog input of the pedal goes through the internal MIDI engine of the Axe-Fx, or it uses a more direct method of using the input's voltage sweeps in controlling an effect block. Anyway, it seems like the latter...