A thread at Ask Metafilter offers advice on buying a GPS. Among the responses:

Just as general advice, I got my first GPS unit last December, and I recommend that you get yours used off eBay. The older models still kick ass, and you get to learn all the GPS receiver jargon and tradeoffs with a lot less outlay. When I first got the idea, I was looking at a very expensive model, but then dropped down to a middling unit and then further to a used one and I couldn’t be happier.

…

It all depends on where you hike. If you hike near large bodies of water, near canyon walls, under heavy canopies of trees, any GPS unit will have errors. Satellite signals either bounce (water, rock) or degrade (trees) in those conditions.

I took a Magellan GPS on a hike near Indian Arm some years ago (Dilly Dally trail). The hike featured a 5,000 climb near a large body of water, in a very cliffy or overgrown area. My altimeter watch and my GPS were 1,000 feet apart in altitude measurement.

I keep fixing to get ready to get a GPS receiver but never seem to get around to it. I think it’s toy overload.