The name of Leica is synonymous with the histrory of the miniature 35mm or small format. The Leica is many things to many people. Almost worshipped by some, denigrated as an elitist rich man's toy by others. I have two Leica cameras, a 1946 IIIc and a 1959 M2. Leica M2. I had to flog some gear, but, finally, I bought an M2. I love it. The smooth film advance, the solid feel, the bright rangefinder, easy to use with glasses and that wonderful shutter sound. Heaven! So superior to the Barnack cameras but many of the sentiments are the same. I do love the finder. It feels as if you are looking right through the camera and directly at the subject. In fact, the camera melts away and become an extension of the eye. It is, without doubt, the best rangefinder I have used. Like the IIIc mention below, the M2 is so tactile, the milled metal, the vulcanite. Love it! The M2 is a huge leap from the Barnack cameras. One word of caution, the 50mm frame on the M3 and the 35mm frame on the M2 are difficult to use if you have, what Leica succinctly calls, 'defective eye sight' (1950's manual). To the rest of us that means you are a genetically inferior glasses or spectacle wearing specimen who should not really be using a Leica in the first place. Can I conclude that only perfect Human specimens can use the perfect camera? Sorry, Adolf, I didn't quite catch that...
Leica IIIc I have wanted one of these cameras for years and when one was offered at a very good price, well, mine it became, and disappointed I am not. It came with a 1936 un-coated heavily scratched and slightly hazy f2 5cm Summar. It just had to be replaced and this was with the f2 5cm Summitar (1949). The venerable Summar still has its place, it's soft and it flares. Some would call it the Leica glow, and yes, the lens has character. It has low contrast (Hello photoshop), but, even so, the sharpness and resolution isn't that far behind the Canonet (below). I just love it. The IIIc is so tactile, the milled metal knobs, the satisfying weight, the feel in the hands, it's so right. It says shoot with me. Even the separate VF and RF is just brill. No light meter, no plastic, no battery, no electronics, no controlling micro computers. REAL PHOTOGRAPHY - YOU in control. It's slow, it forces you to THINK, to STOP, to ANTICIPATE.
Of course, both have no light meter so either an external meter or the use of the Sunny 16 (or Overcast 5.6 if living in the UK) rule will have to do. I would recommend becoming practised at the Sunny 16 rule as it does liberate you from inherently misleading light meters.
So, briefly, how does it compare. I can only make a comparison with my Canon Canonet GIII QL17 and Fed 3 rangefinders and quite a number of Japanese and Russian SLR cameras. Mechanically, the Leica is way ahead. Solidly built with very smooth gearing. You can not only tell by the physically smooth action but also by the sound. The Leica just sounds indescribably smooth. On the other hand, the Japanese and Russian cameras just do the job. Inherently, the horrid and ratchet sound to their mechanism is indicative of an inferior design, build, materials and mass production. My Yashica Mat 124G sounds like its about to make an espresso. The Japanese and Russian cameras feel functional in the hand, whilst the Leica (and to the same degree the Rolleiflex) feel reassuringly pleasurable. The Japanese and Russian cameras just get the job done, the Leica adds to the pleasure and experience.
Optically the same thing applies. I have had 5 Leica lenses, 4 from the R system and 1M. All superb, especially the 50mm f2 that was on my R camera. The bokeh was so good it gave some images an almost 3D like quality that seemed to pop from the print. My M Summarit is a 1949 screw lens with an M adapter, it performs slightly behind the exceptional lens on the canonet. I put this down to the fact that the Summarit, being a 1949 vintage is showing its age with some internal markings. I think when I have a more recent lens on the M2 I will be able to produce a better comparison. One characteristic I find among my Japanese lenses is the fact that they commonly have a very sharp centre and a soft edge (which improves until you reach equilibrium at f11). The Leica lenses on the other hand are much, much more consistent in sharpness across the frame. I actually prefer this.
To conclude I would say that the Leica is a status symbol, its an elitist trade mark, you buy into the culture and the elitism, the feeling. The cameras are better quality than the 'compitition' beyond doubt and together with the best lenses available for the 35mm format make an irresistible allure. BUT, their price point, even second hand, is BEYOND their worth. It is this fact that makes the Leica the afore mentioned elitist rich man's toy. Should you buy one? Perhaps! I suppose it depends for what reason you want to buy. Is it to own a piece of camera history? or the simple pleasure of use? or to join that exclusive club? or is your ego so weak you need to feel better than everyone else by the power of camera bling? or perhaps you feel your photography is so inadequate that owning a make used by Cartier-Bresson or Salgado will somehow make you feel better? One thing for sure, no matter how good a camera it won't make you a better photographer!