My shop uses the Fit Kit as well, but I can't stress enough that it's only used as a starting point. It's good to take some accurate measurements of a person's body (you can get them in the right postures and take measurements quickly that are spot on). That's not to say you can't do that at home. These are a set of items that really are just specialized measuring sticks. They work well for that though.

The Fit Kit also includes some software. That'll do things like recommend ideal crank length, give you a recommendation for pedal to saddle distance and some recommended cockpit combinations. What that means is it'll work out an "ideal" distance from your saddle to your bars (considering saddle position, stem length and drop, etc.). From there you can either adjust your saddle, stem, get a new frame, etc ... or ignore it completely.

Obviously if you've got an special needs for the bike (physical condition, specific frame style, specific event the bike's being used for, etc.) the Fit Kit's recommendations go out the window ... but it's a starting point.