I haven't had the sensor on my new-to-me camera cleaned yet and by the looks of it, the previous owner never either. He had the shutter replaced about a year ago that took care of some problems inherent to the the D600, and he didn't use it a whole lot thereafter.
This picture was taken looking through a dirty window, with a dirty lens (someone else's, my gear is clean as a whistle). The dark spots are dust on the sensor, the larger spots are likely dust on the lens. I thought this made for a good abstract photo.