Husband and I both have one. Makes our job so much easier not having to ask customers for directions to their homes. They're both Magellan units, about 3 years old. Wouldn't want to be without one ever again.
My criteria were:
1. Large enough screen and maps to make reading easy with just a glance
2. Easy to use. When looking at the demo units in the store, I was able to use the Magellans without reading any instructions. The Garmin units required reading the instructions. My husband is not electronics savvy, so this was really important. I'm sure alot has changed in 3 years however. The Tom-Tom was still pretty new at that time, and I wasn't able to find one to look at.
3. I didn't want to spend alot of money. I was able to buy these, on sale, for $100 ea
I also like that it will show gas stations, grocery stores, hotels, etc. That came in real handy when we were working out of state all the time.
There isn't really anything I don't like about them. Sometimes it'll take a route that you think is stupid, which drives hubby crazy. The routing options are fastest (which can get complicated in cities) or shortest, which isn't always faster. You can't tell it to use objective and subjective reasoning, which is what my husband wants it to do.
You might also want to look at how long it takes the unit to reaquire the route when you've missed or ignored a turn. Sometimes it can take a block or two to recover.