I currently own two travel bikes - a Pocket Crusoe by Bike Friday and a Co-Pilot S&S coupled Co-Motion Tandem. I've traveled quite a bit with both (as well as my partner's Ritchey Breakaway 'cross bike). There are advantages and disadvantages to both, but I really like the S&S system. I like it so much that I'm sending one of my older bikes this spring to be retrofitted with couplers. If you have a bike that can be retrofit, the cost is around $600-$900.

Another option (if you are tall enough) is a Ritchey Breakaway. My partner has a Ritchey 'cross bike and loves it.