Very nice accessory for a relatively fast lens (2.8 or less). Somewhat expensive, but excellent optical clarity. A great way to nearly double your reach with only a small penality in light loss.
The converter is very easy to use, and with Nikon's modern lenses it enables all functions; Optics are excellent, and very little is lost in sharpness; The 1.7 is a very nice middle ground between the 1.4 and the 2.0.
The converter is expensive; and a little inconvenient if you are in a hurry