I've owned several C mount lenses from 2.3mm to 150mm focal length. They're very great quality at moderate price, and they're compact and small.
I've tried to use them for some quick and simple DSO imaging by stacking with my DMK/DBK. The results are satisfactory for me.
However, since either DMK/DBK or my existing Lumenera requires a notebook to operate, this renders much of its portability advantage to null. I have been thinking whether they could be used for guiding or not, but with my 150mm f/3.2 sold, the rest were not just up to the task.
I've switched entirely to DSLR and Canon lenses, I'll stay at short focal length work, I won't bother to use a telescope and guiding is also not required.
Today, I have sold my last two of my C mount collections. You know, with the rapid growth of the micro 4/3 market, the price of my C mount collection grew! Not bad, right?