I've owned two GSO crayfords which worked quite well. If you get one that's not quite up to scratch here's a tune up guide:
http://polluxchung.com/gso_crayford_fix/index.html
I like using a crayford on a SCT for the extra fine focus control, but they do chew up a bit of backfocus. The Starlight Instruments 0.8" travel focuser I have is only 68mm long including the part that screws onto the SCT cell, minimising loss of backfocus.(but it's expensive)