By accident I wrapped my duvet cover and a towel around my slippers so they ended up in the wash. I thought they'd be ruined but they look like new now! They are 3 years old and worn every day. The fluff inside puffed up a bit but after wearing for a few mins they feel super comfy again.
So if your ugg or similar slippers are a bit grubby don't buy a new pair, chuck them in the washing machine!