Okay I work at bestbuy and just gonna tell you what I know on them and from when I have ****ed with them.
Tomtom = ****. They are a pain in the ass to use, their maps arent as good as say Garmin or Megallen .
Megallen = Decent, around the same as Garmin. They dont have as wide of a choice as garmin so its harder to find one that suits your needs exactly. And some of the features are harder to use. Like one of the Megallens have a feature you talk to it and it does it, well it doesnt work on entering in addresses, and its really hard to get it to work correctly.
Garmin = My favorite. They are easy to use, good features, they work great, have a good warranty, tons of aftermarket accessories, etc. Ones like the Nuvi 750 are my favorite. They have an FM transmitter, mp3 player, says out the road names out loud where as some others just say turn left in 30 feet, it has blue tooth for your phone and what not. Also ones like the Garmin Nuvi 200 are nice. They are like 300 bucks, dont say the road names, but are just a basic gps that works very good.