I can't add much to what has been said. I have one, and would not be without it. It's SO nice to know you are making a straight cut. With my mitre guide, I could never be sure without dragging the square out and checking it.
As Richard said, 16" will probably do you for most things. But, make a panel sled as well, for those times your crosscut sled isn't big enough.
Sometimes I wonder whether the world is being run by smart people who are putting us on
or by imbeciles who really mean it.