I recently got into M4/3 coming from both Canon and Nikon. Tried both brand lenses in my GX1. The basic adapters work quite well. I got the f-stop adjustable one for the Canon and a standard F mount type of the Nikon since I had a Nikon AFS 17-35.
I have since sold my Nikon gear and still have the adapter. It is made by Fotodiox. It is almost brand new but of no use to me; cost around $24. I sell it for $10 plus $3 for shipping in the USA if you are interested. It also includes the rear cap so you can keep it mount on the lens protected.