Mountain bikes are great for touring. I would recommend getting bar ends to give your hands another position on the handlebars. It's slower than a touring bike though because of the tire resistance and more upright riding position but you will get a smoother ride with the larger tires. I've noticed a lot less broken spokes on MTB's touring vs hybrids or thin tire touring bikes according to journals I have read. I wouldn't recommend a full suspension MTB for touring the road, get a hardtail or one with no shocks at all.
My experience was great on a low end Walmart MTB.