Basically, the app allows you to manually filter your phone calls—weeding out private and restricted numbers and sending those calls directly to voicemail. You can also choose to ignore specific numbers and block all calls within a predetermined time frame. During that time, the service can automatically send text messages to callers with information about your availability, why you blocked them, and/or why they suck.
I haven't tested Do Not Disturb myself, but if you get a lot of unwanted calls and the app works as advertised, I would say that it would be well worth the $3.49 monthly subscription. DND is currently available on a selection of LG, Samsung and Motorola headsets with plans to roll out a version for Blackberry and Windows Mobile devices in the near future. [Riffware and Verizon]