I own the Garmin eTrex Vista and the Garmin eTrex Vista Cx, and I've been really happy with both. The entire eTrex series seems very robust. They're waterproof to 1 meter, so you don't need a separate watertight bag for them if you're outside. I really like the interface. Tech support is responsive and helpful. The battery life is good, and it takes 2 AAs so it's easy to get replacements while you're on the road. Using Li batteries makes it a lot lighter. Some of the handhelds have an issue with the rubber gasket separating after a couple of years, but Garmin has been good about replacing them at no cost with a week or so turnaround time, or sending you a self-repair kit.

I worry a bit about scratching the screen, so I keep mine in a neoprene case with a belt clip and a plastic window for the screen. Unfortunately, I find it hard to read the screen through the plastic, so I take it out of the case whenever I'm actually trying to navigate somewhere, but if I'm hiking it can stay in the case on my belt or pack and still get signal/keep a track of where I have been. Setting a waypoint at the trailhead or your car before you start hiking is always a good idea, but it also leaves a "snail trail" on the map to show where you've been, so you can retrace your steps if you have to.

It can up to a couple minutes to acquire satellites after you first turn the unit on, especially if you're in a car going 60 MPH. This is annoying, but once it gets an initial satellite lock it's almost never necessary to do it again until you power off the unit. If I'm moving in a heavily wooded area, a canyon, or an urban canyon, I sometimes have the satellite signal drop out. If I need to figure out where I am, usually just stopping for a minute is sufficient to get signal back. I'm guessing that this is a limitation of the antenna you can fit in a handheld unit. It doesn't have a connector for an external antenna, but I rarely find I need one.

I would recommend getting the Cx. The color makes the maps a lot more legible and information-dense, and it accepts microSD so you can expand the memory capability. I currently have a 2GB card in mine which fits all of the MapSource CityNavigator maps for the entire United States, plus some extra topo maps. On my Vista regular, I was constantly having to upload new mapsets when I traveled. It could handle the maps for driving from Boston to Portland, ME, or a state or so, but not much more than that. This may not matter to you if you're mostly going to be in the same area most of the time. My old Vista is a serial device, so I needed a serial->USB converter. My new Vista has a miniUSB connector, and comes with a USB-miniUSB cable for data transfer. Needless to say, the data transfer is also a lot faster.

The units come loaded with a basemap of the US, which I found it to be usable but pretty limited. I think if you're going to spring for a GPS, you should also get the maps unless you are only going to use the most basic features of the unit. On my old Vista I used an old version of MapSource Metroguide, and when I bought my new Vista I also bought the MapSource CityNavigator maps. The maps are much better in the CityNavigator product. I don't know if it's a limitation of the maps or the unit, but road routing on the old Vista was pretty crappy. It worked well down to the resolution of a street the size of Storrow Drive, but couldn't get you the last mile to your destination. The routing on the Vista Cx + CityNavigator is great. In head-to-head
competitions with the built-in GPS in Jeremy's Prius, my GPS always wins with the more sensible routings/better time estimates.

I also own some of the Garmin topo maps, some European maps, and some Bluewater charts. All of those work great as well. I don't think you can purchase topo software for the entire US, but the national parks, White Mountains, etc. are covered. Don't take the trail locations in the topos as gospel -- I've been on a few trails where the GPS thought I was ~1/8 mi. off the trail and running parallel to it, and in some cases I found out later that the rangers had moved the trail to limit erosion, although in other cases I think the trail locations on the topos weren't entirely accurate. The city maps are much better in this regard. They are certainly accurate enough to keep you from getting lost, however, especially when combined with waypoints and snail trails. Depending on which country you are in, the maps may be
more or less detailed. Ireland was good; Turkey was poor. I also recommend buying the Garmin maps if you go with a Garmin GPS. Other companies like National Geographic also sell map software, but you can only mark waypoints on the map on your PC and then upload waypoints to the unit, not the map itself. By the way, I think that the Garmin software is only PC-compatible, not Mac or Linux, but that may have changed.

I haven't done much comparison shopping for GPS units. Back when I bought the original Vista, I bought it because of the map software support available from Garmin, and because Garmin has a good reputation. When I bought the Vista Cx, I did it because I had such a positive experience with my original Vista and wanted a better display. I see they also have a Vista HCx out now, but I haven't looked at it.

Remember that they don't work under trees.

All you need is a position display. Anything else is a bonus.