I spent about 16 hours running two dedicated lines from my electrical panel to my listening room. This in the context of a USA split 120 volt 50 hz supply. I have been meaning to do so for about 3 years now and finally got around to it. It was a royal pain to run the new lines (and I have a few holes in the walls to patch still) but the results are very impressive. Overall improved background blackness in the system allowing details to emerge which were previously hidden. It's early days but i am very impressed.
Having said that in the context of a 252/250 system in a prior house I ran a dedicated line and it made less of an impact. So perhaps there maybe some domestic electrical systems which benefit more from a dedicate mains line, then others?