I bought two of these for my Unisaw.  I've seen guys fiddle around and make their own inserts, but I don't want to waste my shop time (limited) making saw parts, so I plopped down the cash for these boys, and I'm really glad I did.  I made one into a zero-clearance insert, which has greatly decreased the already unlikely probability that my Forrest Woodworker II blade is going to produce tearout on the back side of whatever I'm cutting.  I made the other one into a dado insert, which is also zero-clearance for 3/4" dadoes... It makes your dadoes clean and crisp on the edges.  Plus, if you cut a narrower dado, you can use the same insert, it'll just have a little clearance.