I love David Letterman's humour. He is so much better than the competition, Jay Leno. His sidekick/bandleader, Paul Shaeffer, is Canadian, so watch it.
When a show is on 5 nights a week, it's hard not to have a "stagnant formula". But I think he avoids it by onstantly poking fun at the talk show formula and at tv itself.
This came up on thread on LWOS long ago, but has anyone here ever seen the Larry Sanders Show? It was a hilarious sitcom about a fictional talk show similar to Letterman and Leno's.