I agree with most previous members here... What I do with the 3.5, just can't be done with the 2 or the 1.9... I use the 3.5 because it looks so very flat... Amazingly flat without hood... Obviously it's a fine lens, but it's a king when to its optical qualities we add the way it looks...
Most of the time I shoot it at f/8 and f/11 with Tri-X (400-3200) and prefocused... When I want speed, I prefer a 1.4 lens because I'll really need that aperture... In fact, I can shoot the 3.5 wide open in low light: f/3.5 and 1/60 are good for low light at 800 ISO, and f/16 1/500, more than seven stops above that, is normal overcast light... I mean, 3.5 is faster than most of my shooting, and why carry -instead of the tiny, sharp, distortion free 3.5- a big 28 all day (in my case) if I carry a small 40 1.4 for real low light, with the benefit of a second focal length?
And I don't need or ever want a fast 28: I use that focal length for street shooting, not for indoors or low light... I use a 28 to show what surrounds me, and that's done with my 28 stopped down always... With a fast 40 I get selective focus in a way a 28 couldn't even dream about, no matter its speed...
Cheers,
Juan