External GPS unit for Nikon SLR

[OKW Lover]

I've been looking at getting an external GPS unit to automatically geotag pictures taken with my D3100 (soon to be upgraded to a d5100) but don't like the price of the official Nikon unit.

I see several off-brand units available on eBay and Amazon that say they do the same thing at a substantially lower price. Anybody got any real-world experience with these units?