Uwe is absolutely correct - and not just between manufacturers but also within a single manufacturer... even if your manufacturing tolerances conform to six-sigma, one of every 100 or 1,000 lenses will be different in some way.
As for me, I would LOVE to buy more Leica glass but I simply have other financial priorities (a couple in my avatar and I love to travel). My wife bought me my M6TTL and 35mm Summicron ASPH for my 40th birthday. I added a 50mm Summicron DR and I really enjoy both of them. I REALLY want the 24mm Elmarit, but simply can't afford it and don't see a time when I will be able to (large inheritance, perhaps...), so I bought the CV 21mm f4 complete with aux. finder and love it. What a great price!
I was also just looking at another 50mm lens and comparing the Nokton with the Summilux with the Zeiss Sonnar. Something about the Sonnar, faults and all, appeals to me, can't say why but I'm sure the size is part of it - it's smaller than both the Nokton and the Summilux and the price seems ok. I think, however, that I'll try to avoid the GAS attack and not/not buy another 50mm.
On the other hand, now that I've read this thread, I need to take another look at Stephen Gandy's Cameraquest... Oh, nooooo!
Cheers,
Alex