I used to use a mouse but then I picked up a tablet (not for maps originally, but because I wanted to draw again and improve my art and I thought having something connected to the computer I'm always sat at might motivate me more). Now that I've got a tablet though, I'll never go back to using any kind of image editing software with a mouse, I just love it so much.

I did get that initial disconnect waldronate mentions, but found it quickly dissapeared once I got used to it. I used to play a sketch game called BrokenPictureTelephone (it's dead now though sadly) and after a few months of that, it felt like using a regular pen.

If you're doing any sort of drawing, maps or otherwise, I think they are definately worth it. If you're unsure but still want to give one a try then go for the Wacom Bamboo. It's cheap and still nice (a friend of mine picked one up after I got my tablet).