I have been with PhotoShelter almost since they started in business. My website is liveBooks, I was like their 3rd customer or so.
I have always used PhotoShelter primarily as my online archive for selects and for image delivery and sales. Mostly I do ad and editorial sales; a client selects the image or images they want, use the drop down menus to generate a license and then they complete the sale with payment and download the photos. The licensing and pricing is generated by a built-in FotoQuote system.
I also use the system to deliver commissioned shoots. I create private galleries for the client where they can view and download their photos. I do have some "retail" print and personal use download sales too. Again I use the built in systems to create pricing and provide automated printing and shipping from a lab directly to the customer.
There are huge numbers of more ways to use PhotoShelter that I have not even explored. In my opinion it is the most versatile and worthwhile service out there. They have been totally reliable, they are hugely secure and system redundant. And they are hugely successful and have no doubts that they will always be there.