I don't watch anything on a regular basis anymore. If I tune into one of these shows, it's usually because I want to see something specific, like an appearance by the president. Even then, I usually don't learn about the event until it's already happened, and I end up downloading the episode from somewhere.
I'm not a fan of Leno either. And I agree that Letterman comes across as an asshole. A lot of his material seeks humor from someone's humiliation, or so it seems to me. I know a lot of regular people would gladly let themselves get debased to be on national TV. He also has no semblance of sincerity when interviewing most of his guests.