The main reasons I had all these c-mounts were a) a lack of impulse control on ebay
and b) 2 opportunities to buy two larger lots of lenses, one of them with 20 mostly 25mm c-mounts - unfortunately, I never had a chance to pick up a 26/1.1 for a reasonable price.... That was long before the projected release of the Panny 20/1.7, so it was a lot of fun to play with the lenses. I have started selling off many of them (maybe a bit late, the prices are falling again...), but I will definitely keep a few, like one Angenieux 0.95 for its speed, the Cooke Kinic for its unusual bokeh, the Canon for its ease of use, and maybe one or two more. Eventually, I will also get the pancake. But for me, personally, the G1 and the fascination with the combo of old glass and new technology helped me rediscover the fun in photography. I had not looked through a viewfinder in a long time, and going back to the basics, having to manually focus and finding the patience and skills for that again, made this a very enjoyable experience for me.
I don't want to stray too far, but want to mention that I think not only the short c-mounts are interesting. The longer ones are very light and small on the G1 and some make excellent primes, like the Kern Macro Switar 75/1.9. I also like the Schneider Xenars which are totally undervalued on ebay IMHO.
Stefan