Some are decent performers. Many are decidedly not. Leica lenses are expensive because they are made in limited quantities by skilled laborers in high cost regions. Even one of Leica’s flagship lenses, the APO Summicron 35mm f2, is outperformed by Sony’s 35mm f1.4 GM (at equivalent f-stops of course). The Leica lens is impossible to get and costs USD $10,000. The Sony can be had for $1200 and be on your doorstep tomorrow.
There is literally no photographic advantage to a Leica lens versus an equivalent lens from Sony or Canon. How do I know? I had a Leica M11 rangefinder camera and two top tier Summilux lenses. In a little over a year, the body and one lens were back to Leica for service twice due to faulty calibration from the factory. The camera absolutely sucked to shoot with. Colors were horrible and needed a ton of post processing to fix. The lenses were just about adequate performance wise but ludicrously overpriced. The one, and I mean one and only advantage is the small size of the M lenses and M body. Of course you give up a ton for it. Performance, autofocus, optical corrections, reliability. The Leica bodies with autofocus are big, heavy, expensive, and the AF lenses are huge. There’s literally no advantage.
I’m in my 40s and have been shooting since high school. I had always wanted a Leica M. When I was finally able to afford one, I was unbelievably disappointed. It was one of the shortest trips from hope to disillusionment I’ve ever experienced. I will never own another Leica product, there is truly nothing except a size advantage that’s even remotely attractive. The last thing I’ll say is that my photography got worse, not better, while shooting with that god-awful system. There are plenty of reasons I can explain if you’re interested.
This is only true for 50mm or longer lenses. At 35mm and wearing glasses I can't even see the framelines, which renders this meaningless and not useful at all. Considering that Leica's "best" lens (the 35 Apo-Summicron) is in that focal length, it's ridiculous that they can't fix the frameline issues for people wearing glasses.
I can change all my settings manually on my Sony A7R5 just like I could on the Leica, except I can do it better on the Sony because there are three dials that are custom-configurable that I can reach with my right hand without having to take my hand off the grip. Also, why is slowing down and thinking about settings better? Shouldn't the attention be on composition, looking for light, interacting with your subjects, finding new angles, etc? The Leica M makes it HARDER to do those things because the screen doesn't articulate, so you can't find unique angles without lying on the ground, or guessing about focus. With my Sony A7R5 I can pull the screen out and angle it in many directions, allowing me to hold the camera over my head, or down by my feet, very easily. With fast eye autofocus I can also get sharp shots that way. I can't do that with a Leica M.
Let's try a few things. First, on a Leica M, the focus patch is in the very center of the viewfinder. That means that if you want to focus on something, it must be centered in your frame. But a lot of photographs are better when the subject is not immediately in the center of the frame, right? That's a core rule of photography. Even portrait photographers talk about putting the subject's eyes on the upper third of the frame for a nice balance. You can't do that with a Leica M and shoot wide open. So you have this 35mm or 50mm f1.4 lens that you paid between US $5-7,000 for. And yet you can't shoot it wide open with anything besides a central focus composition, because the moment you move the camera, the focus patch is no longer in the right place. If you want to stop down the lens to a narrower aperture to get a deeper depth of field, then you can focus and recompose. But it's also less accurate, and you lose the creative control that you have when you can put the point of focus anywhere in the frame. So the Leica M is bad that way, because you are much more restricted when it comes to where the point of focus can be if you want to shoot wide open.
Then, there's the lack of stabilization on the sensor. A 60MP sensor with no IBIS means any tiny motion of your hands will cause motion blur. Look at how many Leica M photos on the forums are just a little bit blurry. The Leica fans will say "oh it's a classic look." To me it's sad that they justify things that way. On my Sony kit, I can get sharp shots shooting my 50 1.2 wide open, with Eye Autofocus capturing the eye of my subject wherever it is in the frame, and the IBIS means I can shoot handheld at much slower shutter speeds. The Leica M loses every time this way.
Then there's the problem of eye dominance. I'm left-eye dominant. But the Leica M is meant for right-eye photography. The whole "see outside the frame" thing only works if you can a) focus with your right eye b) keep both eyes open while shooting. For me, because of my left-eye dominance, if I held the camera to my right eye I saw a double image that made it hard to focus. If I used my left eye, the camera blocked my right eye so the whole seeing outside the frame thing is impossible. It's an incredibly silly argument for Leica fans to make because I can also see outside the frame by shooting with a camera with a good rear screen and eye autofocus. And that costs a fraction of the Leica M system.
Then there's the problem of parallax. The M focus will change if the camera is angled slightly forward or slightly back. It's enough that a wide-open shot can have the focus point move. So now the eyebrow is in focus instead of the eye. It's incredibly frustrating, especially if you are trying to move your body around to compensate for the lack of an articulating screen. The focus system in the Leica is ancient technology and does not make sense in today's world.
Then there's the issue of manual focus. Yes, you can what's called "zone focusing" where you estimate where your subject will be and hope that you get the focus point on it. But to do that successfully, you have to stop down. So there's no reason to have a Summilux lens that opens to f1.4 when you can't shoot it successfully at that aperture and instead have to stop down. It's silly. Autofocus is just better in every way compared to manual focus-- my Sony kit can focus far faster than my hands could ever move; it will adjust focus hundreds of times per second, silently, and do so wide-open, at any point in the frame. It's just a superior system.
My photography got worse because I was stuck with a stupid central focus patch, no ability to shoot wide open at other points in the frame, no IBIS, no articulating screen, inaccurate framelines, bad to use with eyeglasses, and slow workflows when out shooting.