I'm another that won't upgrade anymore but more for the simple reason I'm tired of dealing with Garmin's sub standard quality. My 478 is a basically a pile of shit. I'm on the third one since I bought the thing 3 years ago. The last replacement the battery stopped holding a charge after 3 months. Amazingly enough, my Zumo has worked rock solid the whole time and I use it most of the time since aside from dirt bike riding the 478 is useless as a working GPS. It's nothing more then a w/e toy.
I'm hoping that one of the other manufacturers comes out with something that has the features and usability of the Garmin's, because I would buy it in a heart beat instead of giving Garmin another penny of my money. Until then I'll just make due with the ones I have, which regarding the 478 probably isn't much longer considering how incredibly unreliable it is.