I have the Canon 50/1.5 and 50/1.9, Jupiter-8, and Summitar in LTM, plus some slower 50s.
The J-8 and Canon 50/1.5 are classic Sonnar formula lenses. Soft and low contrast wide open (especially the Canon), but by f/4 showing good central sharpness and much improved contrast. Some say they aren't general purpose lenses, but I think that only holds true if you're after modern levels of resolution corner to corner. I quite like them both, though I tend to use the Canon more because of the click-stop aperture and the fact that it rotates in the standard Leica direction (the J-8 turns the opposite way, like a Contax (from which the Russians copied)).
The Canon 50/1.9 and Summitar are double Gauss/Planar types, and almost indistinguishable externally. Optically, the Summitar is improved (adding an extra element); the Canon is essentially a coated copy of the preceding Summar's optics stuffed into a copy of the Summitar's body. The Canon shows a lot of softness wide open, and can be swirly and a little crazy in the corners; it really doesn't sharpen up until f/5.6 or so. The Summitar is much more controlled, and has better ultimate resolution, while still retaining a certain creaminess. The Summitar is probably my favorite fast 50, while the Canon is great for those times you want an imperfect look.
I find all of the later Canon lenses - the black-bodied ones - to be pretty big on most LTM bodies, later Canons excepted. The 1.2 is enormous on anything.