Yes, donuts! I often use the 500mm Mirror Reflex at outdoor festivals so that I can get in close without being intrusive. But that also means that I can expect to get those donuts. Some people like them, others don't. But I don't mind them as long as they work.
A99, 500 Mirror Reflex, F8, 1/800, ISO 1600
With a mirror lens though, if the background is too close to the in-focus subject, instead of donuts, the bokeh can get choppy in the high contrast areas, like in the upper corners of the portrait below. But I still prefer to carry this lens around all day at a festival, rather than my heavy 70-200mm with a 2x teleconverter.
A900, 500 Mirror Reflex, F8, 1/400, ISO 400