U.S. Cellular support is now officially available for Project Fi customers. You'll need to update the Fi app to ensure that everything is working properly on your end with connecting to the new network. With this addition, Fi customers will now have three different networks to bounce between to ensure they always are using the best network in the area they are in.
T-Mobile and Sprint were the original partners with Google on Project Fi, and now U.S. Cellular joins as the third. As previously mentioned, be sure to update your Fi app so you can see if this enhances your coverage at all. Be sure to let us know if you notice any differences in the comments!