The original BBC mini-series is a must watch. For me is the best of all of his material turned to a motion picture. Followed by the “The Spy Who Came in From the Cold”.
I can only guess but I think plain old nostalgia must be the biggest reason people are so fond of the original BBC series. It certainly doesn't hold up very well. I think Fry and Laurie's caricature highlights the BBC series’s biggest weakness, its ponderous obviousness: https://www.youtube.com/watch?v=i18bH4XCmHw