I bought this rack after a serious amount of research and I'm not totally disappointed. The thing is easy to assemble and operate, but not perfect. On the product I received, the bottom swing arm ratchet doesn't catch at the low end so I've been forced to push my 26" wheel forward to the 29" setting. This allows the bottom arm to move higher and catch, giving the adjustable screw a chance to grip the tire. Not sure what I would do if I rode a 29er other than return it as I don't think I can MacGyver a fix given the plastic construction.
This problem is most likely only an issue with the product I received, and not a design defect. For me, my next rack will be made by 1Up USA.