My Kindle, loaded with maps and guide books, rides in my handlebar bag. If I need the map (or book,) I stop and get it out. For me, having devices on my bars leads to a clock-watching behavior that reminds me too much of work. Still, plenty of people do ride with phones and GPS units on their bars. The only problem with having a tablet in a map case is that the touch screen functions might not work, in which case you might as well just leave it in the bag.