When I try to use the length vertex group in cycles particles to control the length of hair it mostly works fine, but when I switch to Particle Edit mode all the hairs suddenly have the same length:
The only difference in these views is one is Weight Paint mode (Object mode behaves the same) and the other is Particle Edit mode.
I think this is a bug in Blender but I would like a second opinion before I report it in case I'm misunderstanding how this should work.