My husband and I have been riding a Specialized mountain bike style tandem for years. We only took it off-road once and that was enough. It's heavy, but we've done a few centuries on it. We just got one of these for the stoker seat and I can't recommend it enough:

http://www.thudbuster.com/

If your roads are as bad as ours, you'll want it! The shocks from thuds and bumps seem to travel through the frame and right to the stokers seat post.