Garmin is (correctly, IMO) considered the best (at least, among affordable, consumer-grade units).  You can also be sure that updates will be available for them down the road.  [Unless you live in an area with next to zero growth or redevelopment, you'll eventually want map updates.  All GPS manufacturers charge for these, but some more than others.  Most manufacturers release updates yearly.  Garmin's cost around $85.  OEM units in new cars often cost $200-$500 for updates, and often ship with data that's already several years old.]

Magellin is the other "big name" in GPSs, but it's generally accepted that they don't hold up to long-term use as well, and aren't as well designed/thought out in the ergonomics/ease-of-use department.

Tom-Tom units are okay.  Check into the software upgrades, though.

There are a couple of new, no-name brands like Mio that are cheap, but they currently have very little in the way of support, meaning: few if any available accessories, and not much in the way of manufacturer support.  I would consider these to be disposable, because if they break out of warranty, you'll probably be better off buying a new one.

Finally, there are a number of PDA devices running Windows Mobile that also have GPS built in.  They cost a little more, but they have a lot of other capabilities beyond being a GPS unit.  If you're a PDA person, they're worth looking into.

-Troy

The unit plugs into your PC via USB cable, and you load the data off of a CD or (more recently) DVD.  The full data set for North America (well, Mexico north) is about 1.5 GB.

Some units also accept SD memory cards.  My older Garmin unit stores the data on CF memory cards, and otherwise identical models came with the data pre-loaded on a 2GB micro-hard drive in the CF slot.  I didn't like the idea of a hard drive inside my GPS that would be bounced around on the dash of my truck, but 2 years ago flash memory was still pretty expensive, and lots of people who didn't own computers, or didn't want to bother with them, insisted on fully pre-loaded maps.

These days, nearly all the units come with preloaded maps, as flash memory is much cheaper, so more is included.  HOWEVER, many units don't have the entire map loaded, and instead only include the heavily populated areas.  This allows them to use less memory in the unit (I see that a number of them only have about 700MB of internal memory, which is only enough for half of the data).  Presumably, you can load the missing stuff on an SD card via your PC.  Other units actually have 2GB of onboard RAM, and come with ALL the data loaded.  These tend to cost more, of course.

I do think the BlueTooth feature is nice; with a BT-enabled GPS linked to your cell phone, you can make calls right out of the GPS's "points of interest" database, so you can order your pizza from the car on the way home from work and pick it up, or call the autoparts store for a part as you're enroute, or whatever.

I'm not as sold on the traffic data.  Many units support pay traffic services, either via land-based radio or via sat radio (XM Traffic), which will show you, in certain areas, where traffic is backed up.  That *sounds* good, except that, in the Bay Area at least, it really doesn't help you much, because there usually isn't any way around the traffic for most of the day, as surface streets would take even longer.  It would be nice to have if it was free, but I don't see too much value in it for most folks.

-Troy

I should mention that I have a good amount of experience and information, as I run a DirecTV installation company, and nearly all of my techs have GPS Nav units, so I've seen and used a number of units in the field.

The Garmin NUVIs are the most popular these days, as they're small, include an internal battery, and have all the latest features.

The newer models also have the SiRF III chipset, which is a HUGE improvement over the older units.  It's far more sensitive, so you can actually pick up signals inside a house or under trees, and it acquires the sats much, much faster.  This is very important if, say, you have to make a turn or series of turns just as you come out of a tunnel or out from under a series of overpasses.  With the older units, you can pass your turn before the unit reacquires a signal, which is very annoying at best.

IIRC, it also uses less power.  All in all, a big improvement, and worth the extra few bucks over last year's models.

-Troy