In the same rarified atmosphere where you'll find Leica and Zeiss, perched high as they lord over Nikon and Canon in the hearts of hardcore photographers, also lives a smaller and far less glamorous company by the name of Voigtländer.
Voigtländer has the strange honor of being a provider of "affordable" optics for the Leica M system. I say "affordable" because many are priced at or a little above comparable Nikon/Canon offerings. But compared with Leica/Zeiss (and for the quality) they're a steal!
For a true Leica believer, there is almost an element of shame in spreading the Voigtländer love. As one reviewer puts it: "the hardest part of building a better M mount lens or camera than Leica is getting anyone to believe you" (here). As a result of this you'll find very few good reviews out on the web.
You might want to look at these honest thoughts:
Review of various fast lenses on the R-D1
Review of the popular 15mm Heliar
Erwin Puts' take
Sean Reid's site (need to pay to view)
Unbearably biased but exhaustive comments on the whole Voigtlander line
Short review of the 35mm f/1.2 Nokton
Hold on! Did I say 35mm f/1.2? A mind-boggingly-wide f/1.2 aperture makes this lens the fastest 35mm lens in existence... and it really doesn't suck either (scroll to newsprint tests).
So gaze upon it: