I have used mine for a month straight for navigation up Canada's Inside Passage by boat. It performed flawlessly and, as others have mentioned, you just cache maps to the SD card and you don't need phone service.
It even works well in tree cover, but I have only done a few such trips with the tab. I should mention that when I first got the tablet I was linking it to my Bluetooth gps receiver thinking it would have better sensitivity than the internal one on the tablet, however I was finding weird errors in my gps tracks. When I started using the tablet's internal gps, I stopped having those problems and now get good data every time.