Two of my three GPSs mapping GPSs are also for aircraft use (their primary function), but the mapping software and maps are the same Garmin sells as car-only units. I've looked at a few of the newer car-only ones with my girlfriend, but haven't really gotten too involved with them. We've casually look at the different Garmins and glanced at the TomTom website. It seems that the Garmin receivers are all pretty similar, but they're adding functions like Blue-Tooth capabilities for cell phone interfaces, and a few other things. Their basic functions don't seem to be remarkably different (but again, I haven't really look at them in detail yet).
Having said that: I have a StreetPilot-III, a GPSMap 196, and a GPSMap 496. The software in all three of them are very similar, and all three have user-updatable maps that Garmin sells on CD/DVD. Every so often they come up with a new update for the country that includes their points-of-interest database.
Anyway...I my receivers for the size (easily fits in a jacket pocket, or in the baggy pockets of my shorts), reliability, and features. A small laptop with a decent GPS and decent software is the most versatile, but the newer/faster GPSs put almost all that capability into a pocket-size portable box that easily fits in my MG and Miata.
All my GPSs let me sit down and plan out a route on the PC or in the GPS. Garmin's software will let you sit at your desk and plan a trip, then dump all that info into the GPS to be used on the road. You don't have to do that of course, but it's a nice option.
You can also create your own waypoints ("home", "work", "sis-inlaw", etc). Plug the GPS into your PC, upload the routes/waypoints, and go. Let's say you're out in the middle of nowhere and hopelessly lost - just tell the little Garmin to go "home" and it'll figure out how to get there. No need to punch in an address or fiddle with an on-screen map.
You can use addresses though: all of my GPSs let me type in a street address and it'll navigate to it with relative accuracy - but some of the real power comes into the ability to use their database of "points of interest". Those "points" include businesses, hotels, restaurants, train stations, etc. Let's say you're driving along on a long cross-country trip and you decide you've just GOT to have to have ribs from Chile's. Just bring up your POI menu, select restaurants, enter in "Chile's" and it'll tell you where the closest ones are. If you decide to go to one you just select it, hit a button, and the GPS figures out how to get there and off you go.
The POI functions have been great on trips after getting out of the car too. We'll get into the hotel and look at their little book for places to eat, but then use the GPS - it will also tell you the closest restaurants around you, then you can use it to find your way there.
Two of my three receivers have voice-navigation (the annoying voice Garmin uses on their TV ads). It actually works pretty well. You set everything up to navigate, but instead of watching the map you just listen for it to tell you to "turn right in 1 mile"...or..."exit left in 500ft", that kind of thing. I drove the MG to a show about 500 miles north of here once, and just watched the scenery and enjoyed driving. Occasionally I'd glance at the GPS, but usually I'd just listen for it to "ding" and make an announcement for the next turn. It really made navigating a transition between three roads in-town a breeze (one of those, enter the road here, cross to the left and exit, then merge right in a half mile - type things).
Inevitably people say "I have a paper map, I don't need that"...and they're absolutely right! But, I've had lots of help from mine in lots of situations. I've used them to navigate through cities I've never been before and just drive right to the front door of wherever I'm going without ever looking at a map, a roadsign, or asking for directions (that always seem to get me lost again anyway). It's also great to be able to go on vacation and go exploring, then just hit a button and have the GPS tell you exactly how to get back to the hotel.
Hi Guest!
Hey - did you know if you click on the title of a thread it will take you to the first unread post since you last visited that thread?
but were afraid to ask: 
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smilie in place of the real @
Pretty Please - add it to our Events forum(s) and add to the calendar! >> 
