I received the set (ruler, seam allowance additions, and prep tool) as a gift a couple of years ago, but haven't been very succesful with it.
The ruler has a strip that grips the fabric, which is good, but the seam allowance additions stick out from the side of the ruler and there is nothing gripping the fabric under them. I could see the fabric pulling along with the rotary blade. Part of the problem was, my rotary blade was getting dull so I got a new one, but I haven't wanted to use the seam allowance additions again after cutting crooked fabric strips.
The prep tool by itself doesn't cost too much - about $12. I've used it for measuring 1/4". I keep forgetting to use it for cutting off triangle points! Probably lots of quilters have their own ways of doing things without this tool.