I have used Mothers and Meguiars with less than stellar results, they are often too mild to do much and have fallen apart in my hands.
Zaino, Sonus, and Clay Magic are the best bars available in North America, and Ricardo bars are the best available in the U.K. IMHO. Zaino and Sonus bars are mild-medium grade by themselves and are a perfect "In the middle" bar. Clay Magic has bars that are mild and red bars that are medium. Medium bars will cause clear coat damage that will need to be restored but are excellent at removing heavier fallout, set in brake dust, etc..
Clay Magic has a great kit available at your local auto parts store that comes with a good lubricant. I make my own clay lube per Zaino's suggestion of Z-7 soap and deionized water and have had excellent results, far better than any quick detailer I've used for clay lube. A 32 oz. spray bottle and about two capfuls of Z-7, some water, and you're good to go. :thumbsup