I've found it to be extremely useful. A week ago, I picked up my FIFTH GPS since December 28, 2008. The first three were Best Buy's "Insignia" brand. I had the first one for a year before I found out it was defective from the beginning. Most of the Insignia line are internet-connected, and come with anywhere from 3 months to a year subscription. They have a lot of really great features, but they're completely unreliable (and NOT cheap, even for a "store" brand). After strike three, I ordered a Garmin 765T. When I picked it up, I was pretty excited to be able to have a functioning GPS again...so I thought. My brand new, fresh out of the box 765T wouldn't turn on -- even after being on the charger for almost 15 minutes; I sat in my car in the parking lot waiting for it to gain enough charge to start up. I felt completely cursed by the GPS market.
Then I got my 1390T. So far, it's terrific! I wish this one was on the market when I was first shopping around for one. I frequently make the drive from New Mexico to California (and various parts between), and having a reliable GPS in my car is pretty important to me. This past December, there was a terrible tanker-truck wreck on the Continental Divide. Thanks to my GPS, I was able to make a detour, only delaying my arrival ~3 hours instead of the 12+ hours some people experienced.
I'm a full-time college student, and still use my GPS daily to go to and from school. Not because I don't know how to get there or the alternative routes, but for the forewarning of heavy traffic. On more than one occasion, it's helped me get to my classes on time.
I don't know a lot about the way they work, other than triangulating my position in relation to satellites orbiting above. Many of the brands mentioned on this site are completely alien to me. Several people posted questions about Route 66. My first thought was the first real highway...had no clue they were referring to a brand (or is it the programming?).
Just goes to show I still have a lot to learn about the workings of my electronics. Now if I can just figure out how to create interesting POIs to share, that would be great (with 15 credit hours this term, not really sure when I'll find the time).
I would like to thank Mark and everyone else who has posted POI files and other tidbits of knowledge on here. I'm busy trying to soak it all up so I'll know what's what.
I tested out both units my self and stuck with the 765T. The 1390T was smoother and worked better but it did not come with a quick release craddle. That is, a craddle where the power cord plugs into it directly rather than the unit. I also tested the routing versus some other brands and Garmin just seems to make much better calculations.
For anyone who is not a road warrior, the 1390T is the way to go.