Those are really vague questions.

First, yes I have used them, actually keep it on under the sheets.

Second, As far as how well it works . . . There is a band run under the perimeter on the mattress and 2 nylon bands that run across the mattress so if properly attached it is not coming off the mattress. Also the Velcro pads hold the sheet impressively, I can bend an entire corner of a full memory foam/latex king mattress and then will not lose their grip. Now the down side, they are sheets so they do stretch and can allow some movement and as I said I can actually bend the mattress with the sheets, so there is an element of movement involved, but even restraints attached to bedposts allow as much or more degrees of motion. I have no experience with other under the bed restrains so I cannot compare there.

Third, is it worth the money? That is a personal question and it is up to you.

Finally positions, you do have a greater variety available than with the straps to the bedposts because you have more anchor points, throw in some thigh-cuffs and there are even more possibilities. Honestly, the limits are what you are comfortable with, how flexible you are. I would imagine that adding some of the support pillows on the site would add additional positions. Basically, I'll leave it up to you to find out what you can do with them.