I've used them since they first came out for domestic machines. They're lots of fun. As far as keeping the rulers from slipping on the fabric, I've found that Handigrip made by Handiquilter works best for me. It is clear and has a sandpaper like texture on the bottom so it really holds to the fabric. I've used the shelf liner that gets shipped with the westalee rulers, but it doesn't always grip well enough for me.
Something to remember when using rulers is not to push down on them, if you have a gripping aid like the shelf liner or the Handigrip then all you need to do is hold the ruler with your hand partly on and partly off of it (a finger or two on the fabric and the rest of your hand on the ruler) in order to control the ruler and keep it against the foot. If you have to press down on the ruler to move the fabric then you are more likely to drift off your ruler foot and wear yourself out by tensing up. Also, you will need to stitch a little more slowly when using rulers than you would if doing freehand FMQ.
Rob