Now that I have had the opportunity to create and run an unmoderated user test, I must say that I very much love the concept. In my line of work, its unlikely that I'll be given the time to do quality moderated usability tests, which would require me to sit with participants all day. In this way, I could conduct many usability tests getting large collections of data much more quickly and independently of my time.
There are of course draw backs to unmoderated tests. Fore example, in a moderated test, it would be easier to see issues with the test questions and adjust them early on. If I sent out 100 unmoderated tests, half of them might get completed before I find out my test task were poorly written. Making a correction at that point would render 50% of my results potentially un-useful. There are ways to mitigate this of course. Still I love the concept and I see this as a huge value add in my UX tool belt.