I’m toying with the idea of a handheld GPS unit. Does anyone have experience with any of the models? General advice or other information?
Uses: General London navigation, possible American road trips and city navigation, possible UK and European navigation…
i’ve got one of those built in my nokia 6600, but i have never used it, so, errr, this is an entirely useless response of mine… 
I have a GPS system that works with my hand-held. It has its limitations in that I have to download the actual maps I want to work with, and that it insists on routing me along what IT thinks my most logical route should be (not necessarily the route I want to take). I’ve also found that it can take a LONG time for the satellite to find me (sometimes so long that my session times out!).
I think that, at this point in their development, they’re more a cool toy than a really practical tool, and you can do as well with a good map and a bit of savvy. Perhaps the more expensive setups, such as Magellan, are a different ballgame…I don’t know.
Redwolf
See http://www.gpsinformation.net, and the news:sci.geo.satellite-nav newsgroup, where lots of GPS users hang out.
Thanks Peter!
I’ve got a TomTom wireless (bluetooth) module which hooks up to my iPaq pda… extremely jolly useful on foot or in the car. Door-to-door stuff, great fun. I’ll be using it to find a gaff in Ealing in the not too distant future 
While this is a commercial site below the reviews are quite good.
If you are mainly in inner London then it might be worth thinking about a GPRS receiver, the information is sent by data packet via mobile and is not affected by tall buildings, could be pricey though.
Edited once by Daz to correct typo
Those are the chaps I got my system from.
Not that you’ll be able to read this message before you get on your flight, but we have 2 gps units, one that plugs into a laptop and maps for you as you go.
The first unit gives you your lat and long, which really doesn’t tell you much unless you have a map with that info. Not terribly good for driving, and as an older unit, its slow in locating itself.
The second one, the one that maps, is okay too. It does map, shows you your speed and heading…but it can be a BIG annoyance. The map it displays doesn’t stay with North north, if you know what I mean. I’m used to navigating with a map oriented to the compass.
A really good map is still the best thing. You can get reasonably accurate county maps here at any gas station or grocery store that show every road you want.