First, I don't have an in-the-mast furler, and probably wouldn't -- but mostly because I don't like what it means for sail shape and trimming.

However, There are a lot of them out there and they aren't that tricky.

You need a sail that is really cut for the in-the-mast furler that's stiff enough to avoid problems. It's there that many of the difficulties occures.

Then, while halyard tension is one variable (you can mark for that, assuming you're halyards are stiff enough). The other variables are the boom angle (vertical axis), outhaul tension, and what angle the apparent wind is to the fore-aft axis of the boat.

I think if your sail is in decent shape, you can just get used to it. You have to set-up for the furling action (i.e. reset wtihin limits) if you adjust the vang, outhaul position, etc.

You just can't "force" it too much. If you jam the furler, you'll be sorry! Otherwise, a lot of people like them because they're easier to pull the sail out and put it away.