I wonder what kind of privacy implications this could have (and whether or not we care)? For example, someone could set up a tool to repeatedly poll your service with different parameters over time to get a sense of your daily habits/schedule. (I suppose if this was a concern then you probably wouldn't use this service in the first place.)
Overall though I think this could at least provide a better alternative to clunky out-of-office autoreplies. The sender could make better decisions about where to send an email if they knew ahead of time that their recipient was going to be unavailable.