In my opinion, a GPS is a luxury, rather than a necessity. You should always have real paper maps, and know how to read them, even if they are just a backup to the GPS. I used my Garmin eTrex Vista HCx on a tour in 2008 within the USA, and I found it to be extremely useful for reassuring me that I was on the right road in the middle of nowhere. It just makes routing very easy, and I don't have to worry about missing a turn. Frequently the automatic routing function would take me on the same road that I wanted to go anyway - put it into 'bicycle' mode and it'll avoid highways for you. I didn't need to define routes on a PC and then download them to the GPS; much of the time I could simply pick the nearest small town on my route, and click "Go there". The GPS would work out a good route just about all the time, and mostly (especially in rural areas) the route was very similar to what I had planned out previously on a "real" map. Also, the routing function was useful for getting me across strange towns when I had to go off-route to get to a motel.

Speaking of motels, the POI (points of interest) database was also useful for looking to see if towns had lodging, groceries or other services. However, the POI database on GPS is currently nowhere near complete - there were a few small rural towns where it had no motels, but a quick call to my wife at home revealed something via Google. Often these were little mom-and-pop places that had been around for decades, so I don't know why they weren't included. Hopefully in coming years they will make these databases more comprehensive for the things travelers care about, and omit the things that seem to be completely useless (knitting supplies, lumber companies etc). You also need to be aware that for Garmin, you often have to purchase the City Navigator software (which has the POI database and street-level maps for the USA) separately - though some newer models are coming with it bundled.

Another useful aspect of the GPS was its ability to generate GPX trackpoint files. These were conveniently saved on the SD card with one file per day, named with the date. I could then use these to plot my route that day on Google maps. This might not be so compelling if you are simply using the pre-determined Adventure Cycling maps, but I think it's nice anyway for people to see exactly where you went that day.

GPS is, like I said, a luxury. It should be seen as a convenience, and appreciated as such if you choose to use it. I know how to read a map, and I carry maps, and there's nothing at all wrong with maps (or compass). But truth be told, GPS is just really easy to use - especially in well-served areas like the USA, where the maps are detailed. As long as you view it in this light, as being something that is to be appreciated as an added convenience rather than something you're relying on with your life hanging in the balance, then I don't see any problem with it.

You should probably get a "real" GPS (i.e. one that uses the satellites) rather than the cellphone system, since there are quite a few rural places where there is no cell reception. And bicycle tours tend to take you to rural areas.

I like the Energizer Lithium batteries - using the eTrex all day for trackpoints, a pair of those batteries lasted a couple of weeks as I recall. Obviously you want to avoid the backlight as much as possible, but that's usually unecessary in broad daylight.

I like maps, but now I have used the eTrex, I'll be taking it (or its successors) on any future tours in the US. It's a really neat toy that is maturing into a really useful tool. But, again - always have paper maps as backup, and know how to use them. Electronic gear does malfunction, and you don't want to be out in the middle of nowhere with no idea how to navigate.

Neil