I have one and love it. It is the only way to get a dead on exact speedo after a gear change. It was not too hard to install. A buddy of mine and myself did it in 30-45 minutes or so. It is a little intricate, but that is it. I would suggest one. Oh, and it comes with a manual.
its a breeze. the instructions are pretty good. what the speedcal does is intercept the signal from the vss(vehicle speed sensor) and correct it before sending it to the speedometer. the speedcal has 10 switches and by turning them on or off it knows what your tire size and gear ratio are. sounds hard but dallas mustang has a calculator on their website to that will tell you how to set them.