I've owned four 35 Summicrons over the years and have been happily shooting with one of two version 4s for the last 17 years. It is my go to lens for M photography. While I'll be the first to admit that Zeiss makes a sharper 35 mm lens and even my tiny CV 35/2.5 is a touch sharper (or possibly more contrasty) lens. I really don't care. They both fall a little short in the image soul department.
I love the size, balance and, most of all, the way the 35 Summicron IV renders a photograph. I think it is the best lens to put on a M9.