Sure, you can buy a 50mm f2 lens from an OM Olympus or Fujica for about 20 bucks and get great results, and these lenses are simple and common, but I will say that my Leica 50mm Summicron M Type 4, with a used value of about $1,700, is by far the best lens I own.
You pay dearly for that last bit of quality, and the law of diminishing returns is clearly evident in any Leica purchase; this lens is really something special. It's absolutely tack sharp at f2, where it already easily out resolves the 16mp sensor of the Sony NEX5n. The bokeh is smooth and buttery, and there are no aberrations or distortion.
The camera I most enjoy using this lens on is the M3. I really appreciate the accurate and bright rangefinder patch, which makes getting critical focus easy.
It has a reputation of being the best 50mm lens available at any cost, and I have no doubt that this is true.