Works a treat and you will be supprised how much contamination its brings off that you wouldnt normaly see
They are differant grades of clay bar some are mild some are aggresive,just use the megs one if theres not a lot of contamination,you could you glue and tar remover for tar spots awell
Theres a guide on youtube about claying also check my site www.definedreflections.co.uk there is a link in there