First of all, sorry for the long post. I have some Leica lenses, some CV lenses and one Nikkor lens for my RF, as well as a couple Canon lenses for my SLR.
On the whole, the Leica lenses feel the best in the hand, but only by a bit. The CV lenses feel just as solid, they just aren't quite as smooth feeling as the Leica ones. The Nikkor from the 50's actually feels more solid, but it's ergonomics aren't quite as nice - nonlinear aperture ring that turns the wrong direction, etc. The Canon lenses are great for what they are, but the nature of AF lenses means they are bulkier and lighter. I would say I've never had a problem with any of them, but my Canon 50/1.4's AF just broke yesterday, sitting quietly on a shelf in the closet. Of course, even though I've noted these differences, they all work as advertised and are perfectly usable. They really are minor differences and it's nothing to get worked up over. The CV lenses really are the best deal out there for lenses in my mind, RF or not.
As far as optics go, again, the CV lenses are the best buy. They really are high quality lenses. The two that I have (28/3.5 and 15/4.5) are TINY. Very sharp and easy to use. The only complaint one might have with them is that they are slower. CV makes faster lenses, but they are a bit bigger than their Leica counterparts for the most part. Some CV lenses get a bad rap for ugly bokeh (35/1.4, which is also really small), but to be honest, I don't think it looks any worse for the most part than the acclaimed Leica 35/2 IV. It's about the same size, one stop faster, cheaper, and new. Can't beat that. The 35/1.2 is supposed to be great too. Fast, sharp, flare resistant. What more can you ask for?
In my mind, this is where Leica excels: They make really high quality, fast, SMALL lenses that don't suffer much used wide open. At f/5.6, most lenses are sharp. My Canon 50/1.4 is a nice lens, but you can tell when it's wide open. Same with the Nikkor 50 - 1.4 is usable, but it definitely gets a bit dreamy.
Leica lenses seem to me to be about the same size as competitors lenses one stop slower. For example, Zeiss ZM lenses are great too, but their fastest 35mm lens is f/2 and their fastest 28mm is f/2.8. Both are a good deal larger than the corresponding Leica lens; they are about the same size as Leica's faster versions (35/1.4 and 28/2).
I do think Leica makes a couple of class leading lenses. The 50mm Summilux ASPH and the 75mm Summicron ASPH are very good at what they do, even wide open. I'm impressed with the 28/2 as well. Other manufacturers make lenses that are faster, but I've not seen much out of them that surpasses these three lenses at a given f-stop. And they really aren't big either. The 28/2 is pretty small, and the 50/1.4, while not small, is not big.
I think it's silly personally to have a huge stable of Leica lenses. They are too expensive for what they are. On the other hand, to have 2 or 3 at your most used focal lengths, or even one at your most used focal length, makes sense. Round out your range with cheaper, but really high quality CV lenses. I don't shoot 15mm that much, so when I do, its not a big deal that the CV lens is slower. It's still sharp and small.
All in all, are they legendary? Or something magic? I don't think so. They are pretty consistent, well built, and very expensive. If you use a particularly focal length a lot, shoot RF, and have the money to treat yourself to a really nice lens, then go for it. They won't make or break a picture though. They are analogous to a really nice kitchen knife - you can still make a mean meal with a cheap but sharp knife. The nice one might make the experience a bit more enjoyable and might (or might not) save you 2 mins. It might also last longer. But the food will still be about the same - if you are a crappy cook, it will probably be crappy
