The clay bar is an abrasive. Think of it as super-ultra fine sandpaper. It will remove all surface contaminants from your paint, leaving you a pristine surface for your polishing/last step product. Use PLENTY of lubricant and you'll be left with a paint surface smooth as a baby's behind.
i claybar'ed and meguires 3 staged my car last summer, it took about 5 hours using a buffer, but the result was flawless, the clay bar works wonders, you can hear it pulling debris off the paint as u use it.
I just finished shining up the GT with a Meguiars Clay bar. At first it didn't seem to do much but then when I looked down the panel I could see just how much smoother the surface was. I agree that you can really hear the sound of the contaminants coming off! At first I had a hard time figuring out how much Quik Detailer lube to use, but once I did, it went pretty fast. After the Clay bar I used the "cleaner wax" that came with the kit and it shined up very nicely. I was really pleased at how fast the wax went on after using the clay bar, it really did save time in the end because I didn't have to rub as hard to get the wax off.
I think the finish might benefit from a "top coat" wax or something. Maybe some kind of sealer or something.
I like a shiny car but it sure is a lot of work to get one.
I'd post some pics but the sun went down and its a black car...