I liked Conan back in the late 90s, during the first years of his old "Late Night" show (which was right after Leno's "Tonight Show"), but I haven't watched either of them in years. Never cared for Letterman; I always found him bitter and cynical.
Conan has really hit his stride. Getting Andy back, and being forced to come up with a lot of new material has really energized the show. You can also tell that he just doesn't give a fuck, which is fun. But he does rely heavily on his shtick, which may put some folks off.