I tumble my clad coins. Pennies tumble together and I tumble my nickels, dimes, quarters together. Cleans them OKAY... I use white aquarium gravel and a little soapy water. Heavily encrusted coins take forever, but I clean for about 30 minutes to an hour depending on the make up of the coin.
Pennies turn out okay, some get shinier than others. Most of the coins I find here are so stained with the red clay that there is no turning them back into normal looking coins without polishing them with a dremel. Not worth it for clad in my opinion.