I've been looking into these as well, and while I haven't ridden one, everything I'm seeing says that the coupled bikes are stronger/stiffer than the uncoupled versions. Hopefully someone who owns one will weigh in on this. I also did a quick google check on bike fees, and apparently United will consider a bike a checked bag if it meets the size/weight requirements, which the S&S case should. Of course with the airlines, fees seem to subject to change at a moments notice.