In reply to Simon Pelly:
Personally, no, as none of my regular walls have one, but seeing as I am a bit of a training addict then I would almost certainly use one given the chance.
from what I hear they are very good at pointing out imbalances as you get to do the same move on both sides of the body and it is surprising how different the two sides can feel.
I'd also imagine they are great if you are struggling with a particular hold type as you can just train that one specific hold repeatedly without having to find a route that contains maybe one or two of the right sort of hold.