I've only planned out a couple of days at a time, but I've found my tablet to be very useful. I make sure I have a paper map, just in case.

Couple of drawbacks:

• Need an app with downloadable maps. And, of course, you have to download the maps. I ended up taking a detour that took me beyond the confines of my maps, and took me out of any cell coverage, so I was basically flying blind but for my paper map.
• Need to keep it charged.
• I find it much easier to plan my route on a desktop, so if I'm on the road with just a tablet and modify my route, it can be more hassle then modifying the route on a paper map. Maybe some apps exist or will show up that make that easier, but for now a line on paper is still easier.