I have FC7 expression pedal and here's my opinion (side note: I'm not experienced expression pedal user).
As already mentioned, it can be used for variety of effects, but because the idea comes from organ, it is mainly used as volume pedal. I need to point out, that although it seems to be simple to use it, it's not: it requires a lot of practice. Keep in mind, that it's controlled by movement of ankle, which is not that quick & precise as fingers are. That is, you can do guitar pitch bend, but that will only be for relative slow bending (i.e. slow country or hawaii guitar), but I can't imagine it can be done seccussfully for typical electric guitar (quick short bends) -I think joystick is much better suited for that.
I'm telling that just so you won't be disappointed.
Bogdan