Like any kind of FMQ, there is a learning curve to using the rulers. I have been using rulers and a ruler foot on my domestic machines for several years now and find them to be very useful in helping to reduce the amount of marking I have to do when doing certain kinds of designs. I tend to use a combination of ruler work and regular FMQ on most of my quilts.
You can combine very simple shaped rulers to create more complex designs, or you can get rulers that spin and are good for doing a specific kind of design. It just depends on what you want to do.
Rob