We wanted an easy way to allow users to subscribe to a public list, and I was quite surprised when I searched for Mail Chimp and found exactly what I was looking for. Had it installed and working in minutes.
My only request would be to allow the module to handle more than 1 list. Currently you can only input one list id. I think something like the Acajoom news module would be helpful, where it displays all your lists and allows the user to choose.
Otherwise we are very happy with this module. Great module