Good ad libber?
I'm not saying he is not funny, as I have enjoyed many of his progs. But I maintain that he is not as clever as some people think in world of improvisation.
If you look back at the early Whose Line? you can see he is struggling to be as smart as say John Sessions, Josie Lawrence, Tony Slattery, Mike McShane, Greg Proops, Ryan Stiles, Sandi Toksvig or Jim Sweeney.
You can see him regularly repeating what other cleverer improv. artists are doing/saying. Watch him closely (ignoring the other artists) and you will see what I'm on about.
With HIGNFY, he has this annoying habit of repeating 'jokes' or doing a physical explanation because they didn't get the laugh he expected and even admonishes the audience sometimes for not getting what was a mediocre wise-crack.
Wooden in his presentation would be unfair as he has his moments, but there is certainly a thin veneer of balsa(?) there.
At the end of the day he is certainly no match for Ian Hislop, who I admire for his gracious acceptance of a usual 'defeat'.
So to recap, I am certainly not agin him, I just don't think he is a smart as he is perceived.