I use the Tom Tom One 3rd Edition (Tele Atlas). I like it and it is easy to use. Before I bought mine, I did searches and saw very few bad comments about it -- mostly that there should be more information in the quick start guide (the full manual is on their website -- not in the package). The unit is ready to use out of the box and the maps are detailed -- I even found one-lane gravel roads were listed and named on the TomTom maps. If you want to, you can easily connect this GPS to your computer and download updates from TomTom for free (as often as you want -- every week if you want to). In October a consumer magazine rated it a best buy after testing over a dozen units (there were several units that were rated higher -- but they were more expensive).
It is powered by the car cigarette lighter; the GPS is small enought to fit in your pocket and will run for several hours (longer if you turn the brightness and volume down / off) on the internal battery. The speaker (you have a choice of several different voices) is clear and loud -- there is also a setting to have the volume increase as your speed increases. Businesses are included in the points of interests (gas stations, car dealers, hospitals, parks, police and fire stations, etc.); you can select to show one type of interest, or as many as you like. You can set it to warn you (with a choice of sounds) when you get near one or more of these points of interests. It can tell you how fast you're driving, and can warn you if you speed. When you plan a route, you can have it avoid toll roads, or whatever. There is a "where am I" button that finds you on the map with just that one touch. You can enter a PIN number so a thief won't be able to use it.
This unit doesn't speak the names of the streets (the more expensive ones do). This unit will say "turn left 1/4 mile" and not "turn left 1/4 mile at Main Street." Also, as with any GPS brand, there will be some minor mistakes on the maps, but on this unit you can correct errors, and then you have a choice to share your corrections with other users the next time you connect the GPS to the TomTom website.
It has preloaded maps of the US, Canada, and Puerto Rico. TomTom.com website has a lot more information.