Thanks for the replies. Was looking at a good deal on a used one so it might not be worth it to install something that doesn't work well no matter how cheap it is. Are they meant to only furl(not reef) or are they meant to furl but don't really work?
You still didn't mention which brand of furler it is, but since it's used, I would steer clear.
They are primarily meant to furl not reef. As I mentioned they can be used to reef but because the furler is dependent of the tension of the continuous line to keep it from unrolling it's not as suitable as a single line furler. As Midlife pointed out, it's pretty much impossible to get and keep enough tension on the furling line to keep it reefed when the wind catches it. You would actually have to go forward and lash off the tack of the sail.
Plus, there is usually a pretty good reason that someone decides to replace their furler. If you are serious about it, you would be wise to have it inspected by a rigger to make sure that it's serviceable.
I recently installed a used Pro-furl on a customers boat that they "got a great deal on". It turned out that the drum bearings were completely frozen and had to be replaced.
It still turned out to be a good deal but only because I only charged for about 1/4 of the time I had invested in it.
You would be better off buying a new system.