I used to have one but don't have any Nikon now....
It works well but make sure you add extra diffusion - a couple of Sto-Fens (or copies) are a good start.
I just find on-the-lens macro flashes too inflexible, even the R1, and the adapter may prevent you from getting as close as you want. Obviously (I hope) it will stop you using things like reversed lenses or Raynox close-up adapters. It is a cheap way of getting all the bit, however.