I have one for Bonnie and for us it's a good tool when we're somewhere new and she is beyond excited. I guess the mechanics of the harness mean that pulling is more easily controlled (from the front) but it's not teaching her not to pull, if that makes sense. So, I can keep a hold of my 18kg tornado without hurting myself or her.
That said, we are doing well on our walks at the moment with a long line to allow sniffing and checking of peemail, and then a run around the parks we pass through.
(anyone else have a basket full of contraptions they've tried? Harnesses, long leads, short leads, slip leads....)