The weather resistance is important to me, but I've been wrapping my iphone in a zip-loc bag when out and about, and it's not WATERPROOF in that setup, but it rain-tolerant.

My experience in terms of GPS signal lock, map quality, etc. is the same. I use a Topo app (can't remember name at the moment) and it works the same way - you preload a bunch of maps and it works even where there's no cell service.

I still think that the "dedicated" GPS devices are tougher, but the user interface on the Garmin and Magellan devices just can't hold a candle to the iPS user experience. Android devices are nice too.

As far as battery - there's a universe of external battery options, I have a gizmo that gives me USB ports with just AA batteries, and that works. I also have external spare batteries for the iOS gizmos in my life.