He was funny once, in an absurdist way, then he just became a surly crank, trying to pretend he was still funny. At least he was better at that game than Chevy Chase.
Agreed, he was funny at first. His morning show was very funny. He did things like stupid pet tricks and stupid human tricks. He would put on the Velcro suit and jump against the wall and stick. He made fun of himself by playing the fool. He dropped things off of buildings to see what happened when they hit the ground. He came up with the top ten lists. These were all new on his show.
Then he started to change. I think it started some time after he lost the Tonight Show to Leno. He became a mean vindictive old liberal hack.
Yeah, I watched him occasionally for the first couple of years, but it didn't take long for his schtick to become old.
One telling point to me is how he reacted to Joe Piscopo's mocking of him. Joe would show up on his show with a fake gap between his teeth and mimic him (it was pretty funny). Letterman tried to hide it, but it was obvious he didn't take a dose of his own medicine very well. Joe quickly disappeared after being a regular, and the episode said much about Letterman's character, or lack thereof....