Garmin has a webupdater for Mac.
http://www8.garmin.com/support/colle...t=999-99999-27

With Garmin (as I'm sure with others) there are a couple different upgrade proceses.

The webupdater is for firmware upgrades only and is free.

To update the maps you must purchase a new mapset. New units come with a code for 1 map upgrade that you must use with the next set. I bought mine in 2006 and it came with like NT8. I lost my code so I never upgraded to 2008. 2009 just came out and I had to pay for the maps.

A new mapset will come with new POIs, but I just recently started a thread about how I've resorted to just getting custom POIs off the internet because even Garmin with their market share can't keep up.

I have a Nuvi 660. Great screen. Takes maybe 2 trips to get used to the navigation and interface, then you know exactly what things mean. I couldn't imagine renting a unit unless I always rented from the same company. My coworkers will rent the Hertz Neverlost and the interface just seems so hard to navigate compared to Garmin. Nuvis will speak street names.

The whole fact that the Nuvi 660 has hung around so long speaks for its technology, but i don't use the FM or bluetooth (or MP3 or audiobooks, etc), but I do like the Nuvi because it fits in my purse easily and it ALWAYS in my purse!

I recommend a "friction mount" if you will be using it in many various rentals as I do. The suction cup on the windshield would fall off or never come off.

Good features to have
Spoken Street names
good brand behind it for upgrades (Garmin, TomTom, Magellan)
quick satellite acquiring (my nuvi is slow at this)
Good routing and quick rerouting

Pay extra for spoken street names. You need to focus on reading the street name on the sign itself, not your unit.