I bought the cheapest one I could find. It was from Meguiars. At $30 it wasn't cheap but the others that were available were $50. The bar is about the size of a pack of playing cards. You use 1/3 of the bar at a time. I used some Meguiars Quick Detailer as the lubricant and sprayed it on the paint and onto the bar itself. You don't have to use any force...the bar does the work. After a few passes, you can really feel the difference. When one side of the clay is dirty, you just kneed it to expose a clean section.
The bar will remove paint overspray, tree sap, dried bugs, water spots exc... You need to wash the car first to remove as much of the contamination as you can to begin with.