It's a really nice lens, very advanced for its time. It's a Biogon copy, before Biogons got a massive redesign after the war.
Quite fast, not much vignetting, but the price for the latter is a bit of pincushion distortion. Obviously sharper in the centre than the edges, and sharpens considerably by stopping down.
The only other option that you have is the postwar Biogon 35mm, which is better (less field curvature) but much more expensive.
Aperture ring is a bit of a pita if you like to change it very often, it gets easier if you attach a filter.
If you feel tempted to look up test shots online, disregard anything digital. Literally everything. The J-12 rear element sits very very close to the film (sensor) plane, the closest that I've seen in a lens, can't even fit in postwar Contaxes that have the same mount. This causes the sensor to play up and show colour fringing and blur which is *not* the lens' fault.
Which 50mm are you getting btw? The Jupiter or the Helios?