Steve and I have a Garmin and have another one on order for his new car. Garmin's consistently rate well with Consumer Reports and get strong reviews on Amazon - so we stuck with them. The Nuvin750 we just bought is supposed to do routing for multiple destination, which should be a useful feature if it is implemented well. If you have a bunch of errands you can enter each place and it will order your route for you to maximize efficiency.
TomToms, which we don't have, might appeal to your inner geek because there is some open sourcing with it. So people can update maps and stuff like that. Don't know much about them or how well it works. But I could see that appealing to you.
Magellans might also be OK - they are used in Hertz car rentals so you could try it out by renting a Hertz with Neverlost for a day. But Garmin is considered superior to the Magellan by most accounts.
One thing to check is what maps are preloaded in the GPS. Ours has US and Canada. Some GPS' systems have very little loaded in. You want detailed maps for all of the US at a minimum I would presume. Some will only come with major roads and freeways and you have to pay for downloads of maps of more specific areas. Some models might come loaded with European maps, which means when you go to Europe you could use them to walk around and find places (the Nuvi 350, our 1st model, can be used that way but we would have to pay to download European maps).
I've been frittering around the edges of this issue myself, and have largely decided on a Garmin Nuvi because not only are they a good GPS system, but they have additional features which most don't. I'd go with one of the widescreen ones, like the 660 or whatever their current studly model is.
We've had a not-high-end GPS (Mio) for a few years now. Although it's not one that gets regular updates, it has proven it's worth many times over. We're looking at upgrading to a Garmin this year. From research and reviews, it seems that no Garmins are ever rated poorly, you just look at features and prices and find some spot on the scale you are comfortable with.
If you decide against Garmin for some reason, only the cheapest of Tom-Tom's ever get poor ratings. Anything over $200 is well-rated.
My next step in research is poking at tech support and warranty relations. I've never run into anyone who's had a warranty issue that was not resolved simply by returning it to the store they purchased from, but I want to know how they rate. Murphy's Law and all that.
Good luck. I'm quite interested in your decision-making process.
I'd recommend a Garmin, because I own stock in them :-) But I'm not just a shareholder, I'm also a customer. Unless they've improved the MP3 interface, don't bother getting a model just for that feature. Bluetooth hands free is neat although the volume leaves something to be desired (but that could be due to an apparent decline in my hearing).
We have a Garmin Nuvi 350 with which we have zero complaints. I have used other systems and have managed to figure them out, so I am not sure there is a huge difference from one to another.
One accessory suggestion - purchase a bean-bag sits-on-the-dash mount and throw the suction cup out ASAP. The suction cups, ummm...., er...., suck. Any of them will leave crap on your windshield in short order then stop working due to the missing crap. The bean-bags are the stone-cold nuts.