I never liked Letteman from the first day he came on the air. I have never found him funny. The way he would position his desk hire than that of the guests.
I *used* to find his show to be *very* funny--much funnier than Leno. Back when he was on NBC, I watched the show a lot, and it was sharp and clever and witty. Even after he moved to CBS, it was still mostly funny for a number of years. The last five to ten years, increasingly less so, and thus I hardly ever watch at all now.
Letterman, Keillor, even Franken, who at one time was funny--the more political these people let their act become, the less funny they got.