I busked on the London Underground for a couple of years on and off and largely enjoyed it. There were times, though, when it the dust and monotony got tedious - but that was maybe my fault for having an hour-long set that didn't vary much and playing it on a rather loud melodeon!
What made me stop was the growing number of unpleasant and violent types who seemed to have a problem with busking. After one afternoon when I genuinely feared for my safety I simply gave up and haven't been back.
Back then, when it was still illegal, some Underground staff tacitly encouraged busking. One told me that it made other people feel safer, especially in some of the longer footways like Green Park and the museum tunnel at South Kensington. Even the Transport Police were generally pleasant - they'd ask you to move on, but never once was I threatened with arrest. Sadly there wasn't much they could do about the average, ticket-holding psychopath with a downer on buskers!