I got a Garmin Streetpilot years ago and it has never let me down. I also have a ETrex Vista, which has been a good little unit. For Christmas, I got upgraded to a Nuvi, and although I haven't used it much yet, it appears to be a great unit. It is amazing how much the technology has improved from the Streetpilot to the Nuvi.
I am a big Garmin fan, as Iowabowtech said, the customer service is pretty good, the software is easily upgraded, and the maps are just as easily upgraded, for a fee of course. I have no experience with any of the other vendors, but I don't think you can go wrong with a Garmin. I will say that the competition has been good for the consumer. For what I paid for the Streetpilot years ago, you could buy a couple or three of the lower end Nuvi's now.