The ability for me as an admin user, access all of my customer's info without having them login or be logged into our system! It allows us so much more flexibility when it comes time to send out emails/marketing materials etc..
I wish there was one place that we could go if they were no longer with you but still wanted their information saved (i know this isn't possible). We are able to track who has opened what email from where which helps immensely during sales calls - makes things run smoother than before!!