Did first week of November last year; it was great (weather and flows were fine -- 1.5K at Greendale when we put in at Gates). Obviously cool nights and early mornings, but delightful during the day. That said, I did wear my drysuit (13' cat solo) -- didn't want a wet swim in November (didn't swim, though). The little bit of rain while on the river was of course no problem with the drysuit -- rain was timed well -- mostly while sleeping. Saw nobody else -- oh, except a group we passed getting it together at Island Park on our way out.
Of course, weather is hard to predict -- Did April Yampa in 2014 and it was really nice; this year wet and cold most of the time -- even a bit of snow.