Ishinguy - if you are going to use one of these machines, then make sure you use one that supports the knee. IMO they are not much good - at least they were'nt for me and that's the crux of the matter, use what works for you as everybody is different. I personaly use a rope on a pulley mounted in the ceiling - my flexibility came on leaps and bounds when i started using this some time ago and i recommend it highly. In any case, experiment and see which suits you.
Regds and good training.