I got into it from the foot-powered side. Bought a Garmin Geko 301 (very small/simple/light B&W unit) for mountain hiking, and it eventually replaced the cyclometer on my bike, too. Knowing your altitude, heading, how many miles you've walked/biked, the security of a "bread crumb" track so you can find your way back even if you got off the trail or lost daylight... all of it is invaluable.
When I had to make a 1,000+ mile round-trip between Virginia and Connecticut, I decided a moving-map display would be useful. I got a Garmin GPSMap 60CS (later upgraded to 60CSX) because it's a good compromise between hiking/biking and car navigation. The screen's a little small for auto use and there's no voice prompt, so I wouldn't recommend it to anyone interested primarily in road navigation. Even so, I basically
never go anywhere without the thing anymore... it's just too darned useful. (One example: Need a Shell station? Bleep-blip-bloop, there's a list of the closest locations; pick one and bam, turn-by-turn directions! Easy as pie.)
My wife was always mildly curious about my GPSr though I think she saw it more as a "man toy" than anything. But after she got lost in New York (ouch!) she expressed a newly-found and sincere interest
and wound up buying a TomTom. "Best money I ever spent" is how she rated it after owning it for a year.
I'd recommend one to anyone who does a fair amount of driving, if only for the convenience/security factor. For gas conservationists it's a great tool: Need to get to Sam's Club from "here?" No matter where "here" is, your GPSr will map out either the most direct or the fastest route. Tailor it to your driving style, too: At least with the Garmins, you can tell it how fast you drive the interstates, country roads, city streets; whether you want to avoid the interstates; what you're driving (lie and tell it you're on a motorcycle or bicycle and it'll give you different routes, some of which can be perfectly suited to low-speed P&G). And so on.
Every bit as indispensible as a ScanGauge, if you ask me.
Rick