Letterman from 82-95 was just awesome stuff. Best talk show ever.
Letterman from 96-00 was okay, but you could tell he wasn't quite at his peak anymore.
Letterman from 00-05 was pretty dull and complacent, didn't have the energy, creativity or edge anymore.
Letterman from 05-current is just someone who either has totally lost it or just doesn't give a fuck anymore.
I used to love the stuff he would do on the street. But, he rarely,if ever, leaves the studio anymore.
I think part of it is the heart attack. I think it may have made him a better (less cynical) person, but it also kinda killed the vibe of the show. And, I'm sure age and doing the show for 30 years led to that decline as well.
And the thing is I didn't even see his best years live (as I was born in 86). I've seen a lot of it via youtube, and a DVD collection I bought. Comedy usually doesn't age well. But, Letterman was that good for over a decade.