I've had a 400T for close to a week now. I've been using a Garmin Legend HCx. Some things I like better with the Legend.
The paperless caching is great. I like being able to use the field notes. The field notes really simplify logging the caches.
Some minor things I don't like but I'm getting used to them now: After finding a cache, and requesting to find the next closest cache, it goes straight to the map. I haven't a clue what the cache is. I need to back up a few steps and then go to the geocaching screen. It would be nice if that was simplified. When selecting a cache to find, and selecting go, it goes to the map screen. I would prefer it go to the compass screen. The HCx would go to the compass for Geocaches and the map for waypoints.
I really like the touch screen. Navigation is simple, though some tasks take several steps and it seems it could be done with less.
I had to project a waypoint today. I was able to figure out how to do that on the Oregon in about 10 seconds. That was cool.
As others have said, the screen could be brighter. The angle you hold it and look at it seems to make a difference. It seems fine with the backlight on though.