Just make sure you have a good lint trap/filter on your washing machine, or you will have a drain clogged with felted wool!
You can prevent this mess and potential damage to your machine by putting the sweaters in zipped up pillowcases. Add some old towels or jeans to increase agitation.
This is a kitty I made from a felted sweater. It has catnip inside and was a gift for a real kitty who was undergoing chemotherapy. The recipient's owner told me it was a hit.