Spent six days in the park last week; three at Old Faithful, three in Roosevelt. Lots of great wildlife sightings along the way, particularly in Hayden Valley and Lamar Valley. Spotted a few black bears, two grizzlies, elk, pronghorn, bighorn sheep, osprey, and, of course, lots and lots of bison. Edit: also marmots!
Big fan of this new lens. 75mm is a great focal length on a film M, but I wouldn't have taken my 75 Ultron to a ballgame -- the bigger the lens, the more likely security won't let it through. This one, though, is meaningfully smaller. Slower too, of course, but I don't think I need that extra stop.
I usually carry a film camera and a GR, but for spring break I went with just GRs, plus the 75mm teleconverter for the IIIx. It was great traveling so light. And with 28mm, 40mm, and 75mm focal lengths covered, it didn't feel like I was compromising anything.