I downloaded the trial version of BackCountry Navigator. I'm in the process of downloading the entire Olympic Park from mytopo and from what I've browsed of the device....it is pretty slick.
I think the biggest complaint I've seen with these gadgets is battery life. It is a mini-computer so it can suck a battery dry quick if you don't use it sparingly. Use it in Airplane mode and just turn on the GPS when you need it to acquire a location. Turn it back off and keep all the cellular features off and you should be able to get reasonable use from it.
I'll update once I've actually used it in the field but from the living room, I'm pleased with how it works. I'll still carry paper but this certainly gives me another reference when I'm going off-trail. It also doubles as my emergency phone, camera, emergency flashlight and night time entertainment. Well worth the extra 5oz.