My feelings toward both Leica and Sigma have evolved over the past decade. I used to see Leica as an overpriced brand with a cult following that produced strong products, but that didn't justify their prices. Conversely, I saw Sigma as a cheap lens manufacturer that produced affordable alternatives to the lenses you really wanted. Then, two separate occurrences changed my opinions of both.
With Leica, I visited their office in London and tried out their cameras and lenses. Although I retained the belief that some of the equipment was too expensive for most to justify, I did see the "magic" that many adore. Shooting with a Leica camera and lens was a singular experience in ways it was hard to articulate. With Sigma, it was the introduction of the Art range of lenses that showed me the quality they can produce, making them not just a brand of cheaper alternatives, but highly desirable in their own right.
In this video, Alex Berrar compares the Leica Summicron-SL 50mm f/2 ASPH. with the Sigma 50mm f/2 DG DN Contemporary. The latter is one-third of the price of the former, but does the quality correlate?