I have the 20 and like it and it is quite useful, but the thing I do not like is how much play there is in the upper rail, not sure how to make it better, but if you have things lined up and it is a little bit cocked, the top piece will move and could cause you to start pressing out of center.

I have thought about putting some tefflon baring slides on the top piece to remove some of the slop but have not got round-tuit

other than that, very much worth having

My dad built one years ago, same basic design except the jack pushes on a plate and only the shaft moves in a close tolerance slide this is a better design, but I can not exactly explain it...