I know this is an older post, but since I really like this topic I thought I'd jump in.
I agree with everyone who has said that Watson is not at all a stupid man, nothwithstanding the ACD interview. The written stories themselves and the later BBC and other radio programs consistently depict Watson as a competent lay evidentiary thinker, albeit not a spectacular one, as well as a respected physician. (I find this interesting, because at least from what I've read ACD himself was only a mediocre doctor.) Watson is like the everyman reader who enjoys trying to solve the mysteries but, like everyone else on the planet, doesn't come close to possessing Holmes' acumen. And Watson is certainly no less competent than Scottland yard detectives Lestrade and Gregson, whom Holmes describes as the best of a bad lot. Although the Bruce radio depiction of Holmes at times make him appear less than brilliant, much of that is attributable to his semi-incoherent mumblings and not his actual approach to problems and situations.
I think Watson is an interesting character study in and of himself. Unlike Holmes, who at least arguably is a progressive social thinker as reckoned in late 20th Century terms, Watson is clearly a social conservative who strongly supports the British peerage system and who frequently addresses perceived social underlings as "my good man" and the like. At the same time, Watson is clearly a humane and compassionate individual, as well as a fiercely loyal friend to Holmes. Watson is a man of character, despite his obvious penchant for the opposite sex (which may have been intended by ACD as a contrast to Holmes aversion to most things female). We can all relate to Watson in ways we can't relate to Holmes, which makes his choice as Holmes' sidekick a brilliant one.
