To me they are hugely funny, and the one-liners are brilliant. I have to assume that parts of the show were written well beforehand, but hope that some were genuinely off the cuff lines from very funny people like Willie Rushton, Tim Brooke-Taylor, Graeme Garden & Barry Cryer.
Whoever wrote Humph's lines is\was a genius - I have a sneaky suspicion it may have been Barry Cryer. Some gems can be found here.
The last word has to be the closing line of Humphrey Lyttelton on his last ISIHAC show......... "And so as the loose-bowelled pigeon of time swoops low over the unsuspecting tourist of destiny, and the flatulent skunk of fate wanders into the air-conditioning system of eternity, I notice it's the end of the show..."