In my experience the Jupiter-8 can be an excellent and cheap lens, however, the focusing issues people mention are real. You may need to shim the lens to get accurate focus, particularly close up, as they aren't built to exactly the same standard as LTM-proper. That said, if your example does focus accurately [or can be made to do so] they are pretty good. I've had a couple, and both were excellent. Neither focused completely accurately wide-open and close-up on my Leica body without shimming.
I use a Summar on my Leica body. They are, perhaps, a bit more of a 'character' lens. Mine is coated and sharp and with decent contrast stopped down, but fairly soft and with wild out-of-focus areas at the widest couple of apertures. They can be found very cheaply. Perhaps not as predictable or standard in rendition as the Summitar, say.