Well, compared to the current 35mm 'lux the Nokton is the larger lens, but neither is a pancake by any stretch of the imagination. I can't say the difference in lens size is really that big of a deal to me. Both are too big to fit into a pocket with the camera. Both are small enough that I can carry them on a camera all the time and not have it be too obtrusive (at least relative to my DSLR zoom lenses).
I guess for me it's like saying the new Mini cooper is a bigger car than the old one. It's very true, and to someone used to the old one, the new one is significantly bigger. But at the end of the day, they both squeeze into parking lot spaces pretty much the same, at least for me.
I did see the 40mm f1.4 and that certainly is a very sexy/small lens. Personally, I don't really feel the need to go small, but maybe I just like the size of the camera with the Nokton on it. It just feels like the right size to me. I have the 12mm f5.6 which is a much smaller lens, and I dunno, the lens almost seems too small to me. Again though, I'm coming from a DSLR background, so all this stuff seems rather small to me to begin with.
I do use a multi-coated UV filter. I used a regular single coated UV filter, but it does cause internal reflections (flaring) when strong light sources were in frame. I noticed when I removed the filter, the flaring would go away. I hope the multi-coated one does a better job than the single coated one, but it's too early to say how much better it performs. I use B+W but only because it is the brand that has been recommended to me by others.
I use the optional vented lens hood and even with it, the lens does block out a bit of the lowest right of the frame (maybe about 15%.) I don't find it's a big deal for me.
I only recently got into rangefinders (about two months actually), but with the R-D1 and the Nokton, I shot more shots those two months than I did in two years of owning my old Canon system. I find I want to carry it all the time and my shooting has improved as a result. I'm so glad that I tried the R-D1... I hope you have as much fun with yours as I have with mine.
Best regards,
-Jason