It gets a bit of a bad rep, mostly, I think - because people want to compare it directly to the Noctilux. Which is fair of course, but a Noctilux it's not - fast, yes. But it lacks that Noctilux mojo wide open and the bokeh is a little unpredictable. Still, it's a decent lens especially stopped down even a little.
As for the focus shift, I never really notice it in day-to-day shots. It happens mostly from f/2.8 to f/4 - and at closer distances. Of course, this is a pretty popular zone for shooting. If you scrutinize, you can see it in some shots. If you're trying to nail focus on something thin it could be an issue, but more three dimensional subjects not so much. Of course, a fast lens like this is often shot closer to wide open where it's a total non-issue.
Check out my
gallery from the Mermaid Parade 2011 in Coney Island. I shot it entirely with the 50mm f/1.1 Nokton on the M9. Despite the apertures listed, the majority were at f/4 except the obviously thin DoF shots which were mostly f/1.4.
Honestly, unless you plan to shoot this lens closer to wide open more often than not, there are probably better lenses out there - not so much optically, but size wise. It's not a small or light lens. It also blocks a fair bit of the viewfinder.