I think the general feeling my most is that it works so-so. With practice you can get it to do a 90% job. Problem is, what do you practice on?
Silver is one of the most difficult colors to do, and I would say Langka works better on silver than most other colors.
If you're going the Langka route, I recommend allowing the paint touch-up to dry several hours or even over night before using the Langka to level. Be sure to have a fine cut compound and a good polish to finish the job.