Jez,
I use the Roland A-800 Pro controller and really like it a lot. I don't know how much different it is than the A-500 Pro. Overall the Roland A-XXX pro series has been a reliable and well built product. I use my A-800 Pro with vArranger and it is possible to totally control the software via the controller - very much like you do with a full fledged arranger. The key feel of my controller is pretty good and probably comparable to most TOTL arranger keybeds.
Hope this helps.
Deane