I purchased the 400t a few months ago, messed around with it, then returned it after coming to the realization that (1) most places I go are marked well enough to get by with a good map and compass, (2) it's heavy, (3) it's bulky, (4) it's expensive, (5) it required batteries (see heavy and bulky), (6) it was technology that could break and therefore I considered it only mildly reliable, and (7) I really hate thinking that I'm lugging a bunch of technology with me through the woods where I went to escape it in the first place. I also wasn't terribly impressed with MapSource, the Garmin mapping software, as many well known hikes weren't on it. I also didn’t find it to be very detailed. Other free web sources got me the TOPO’s I needed, although I’ve been thinking about getting the National Geographic TOPO! as frequently mentioned on this site.
From my short experience with it, the menus looked exactly as they did a few years ago. The only difference was a screen in color and a better antenna. With that said, I found it to be fairly intuitive once you get the idea. If you want a quick demo, and perhaps a potential helpful hobby, go to your local Sheriff's Office (or Police Department) and talk with their Search and Rescue folks. They always use GPS' on their searches and no doubt wouldn't mind helping you understand yours if it would forgo a need for them to come and find you if you got lost in the future. You might also find it to be an experience worth doing (i.e. volunteering for SAR efforts). I believe local community colleges also offer similar courses for relatively cheap and most camping stores (REI, Dicks, Blue Ridge Mountain Sports, etc.) offer seminars on this kind of thing too. Books will tell you how to use it, but they aren't always terribly clear and seeing it is almost always better than reading about it.
Good luck...from another Virginian.