I grew up with Letterman. He was always my favourite of the two. Always found him so much better than Leno. I don't think he was so much funny as he was entertaining to watch. Some of his jokes would make me laugh, especially in the 90s when he was more proactive in doing things like going out to the Hello Deli. He certainly slowed down over the past decade and I think that kinda hurt the appeal for the show. After watching the last week of shows (I don't have cable and haven't really seen late show in about 5 years), it was time for him to retire. His self-mockery humour seemed more bitter than light-hearted like it use to be. Maybe because he knew the show was ending but it seemed like he was just getting tired of it all over the past month. His jokes were also very dated I noticed but that's to be expected I guess.
The last show... I felt it could have been a lot better. Even his final signoff seemed quick and heartless to me rather than the emotional moment it should have been. Then the Foo Fighters were playing up till pretty much the last second and I was really worried they would end with just that. I hate that empty feeling of incompleteness when a huge event ends like that. They did show him from the back waving to the crowd but... meh. :\