First.....
I would NOT perform without my expression pedal.....also my sustain too....all my registrations are basically calibrated to my way of playing, and that is using the Volume/Expression pedal.
Coming from an Organ background, pedals also with footswitches in the sides are a second nature for me, I like the Ketron volume pedal http://www.ketron.com/itemdetails.asp?mod=TP
To bad that has a proprietary connectors, non standard that would allow to be use with other instruments of other brands.
I loved to use this pedal when I had my X1HD and the SD1, both keyboards could be controlled with just one pedal, I really like it a LOT, besides been almost indestructible.

I always have use "volume" pedals, and for advise, I had BAD luck with the Yamaha FC7, I must have a half a dozen that came apart eventually, so far, I would NOT recommend this pedal to no one.
I also had tried the Signal volume pedals like the Korg XVP10, that also has a dynamic control potentiometer, they work OK, until start making some noise after a couple years, very well constructed and heavy duty too.

Believe or not, I had better luck with some old GEM volume pedals, but, eventually they go bad, since they are the cheaply made.
I also purchased the ALesis F2, similar to the Korg but it has a half way, then a spring action gets the volume pedal to that half point, this is by the way, also a signal pedal with the dynamic control. I did not get use to it, so is in a closet.

I hope somebody makes a USB Expression pedal that use a photocell to control the Volume with a LED instead of the conventional light bulb, much like the old Hammond organs, to ME seems the best dynamic and durable option just bringing it to the 21st Century.