HC KV tester
 

http://hobbycity.com/hobbycity/store...for_BL_motors_

Has anyone tried one of these? How does it work? Would there be any value to getting one or is it's use on RC cars too limited to be useful?

It doesn't appear to be a data logger as such so you can only use it when the car is on a stand and that would give innaccurate readings for realtime use.

Neu's got one too:

http://www.neumotors.com/store/page9/page9.html

They look identical.