The interface is pretty simple and straightforward. After creating an account on momento360.com, just upload a 360 photo or video and it appears in 360 on your screen (you can also maximize the view). It recognized a test file from the Nikon Keymission 360 (reviewed here), even though the Keymission has incomplete 360 metadata.
On a smartphone, you can view the image in cardboard view by turning your screen sideways, maximizing it, then tapping on the Cardboard icon.
Your uploaded photos and videos can be shared by tapping on the share button, which will send an invitation to your intended recipient.
Thanks to reader Pepe Vazquez for bringing this to my attention!