Harry Knowles isn't exactly a film critic despite writing reviews of films. He's not exactly a fanboy as he actually engages professionally in his chosen vice and has enough fame of his own to have his own fanboys. He's not a journalist since he largely posts rumours, unsubstantiated previews, highly subjective personal opinion, and hearsay. However, he is the guy who runs the film site Ain't It Cool News. A title that more or less perfectly describes the site (assuming one knows it deals with film).
Harry is based in Austin,TX where he has been running the site since 1996. Apparently the layout has not been changed since then as "headlines" scroll down in garish hues and 20pt fonts accented by an excessive degree of capitalization and punctuation apparently stolen from what the warez d00dz with taste consider to be going a bit too far. Harry's a rather large fan of "typical geek films": e.g. Star Wars, Lord of the Rings, The Matrix, etc. Think of a fanboy's dream come true. That's about it. Harry uses his site to post stories often sent in by others and accompanied by a slight bit of gushing from either himself or another one of the regular writers on the site. Saying that these rumours vary in their levels of validity is like saying that the leaning tower of Pisa looks a bit wobbly. I'm not here to talk about the site though, but it's rather hard to seperate the two.
Harry occasionally writes reviews as well, which again vary in quality greatly. Sometimes he manages to really get to the heart of the film as he uses a highly subjective style that as he puts it "doesn't begin and end with the opening and ending titles". This means a review of say... Charlie's Angels is as likely to discuss how enraptured he was with Cameron Diaz's ass as it is with whether the film was any good or not. His opinions are also quite unusual and often tend towards psychotic. Marked examples of this include loving Blair Witch 2 and continuing to defend it after it had come out and been universally panned to supporting the Star Wars prequels as being fit to bear the name Star Wars. The comic strip Penny Arcade put it rather well by stating that if one were to put money in a jar every time he's wrong about a film you could send your children to college with the accumulated funds. I believe this was after he said Minority Report was a terrible film not fit for human consumption in any capacity. The variances in his reviews have eventually come to people suggesting that he's being paid off in some form or another, but I'll leave that issue to people with more money to fight libel suits. Nevertheless a glowing quote from Knowles has become increasingly common on film trailers, posters, and video boxes.
As far as non-web appearances goes he has managed to make several rather high-profile showings. He appears in a cameo in Robert Rodriguez's film The Faculty (Rodriguez being a friend of his) in about two scenes as a film teacher. He's the redhead who looks like the before picture of Jared the Subway spokesperson. He also guest-hosted with Roger Ebert after the death of Gene Siskel before Richard Roeper took on the position. As well in 2002 his book, titled "Ain't it Cool?", was released though I've not read it. His site is worth reading occasionally to keep abreast of film news as it does contain interesting material every so often. Sadly this and the site's general popularity has also led to him being frequently cited alongside the words "internet", "film", and whatever else might be applicable making people associated with either term cringe in fear and revulsion. His birthdays are celebrated with the charity film festival butt-numb-a-thon at the Alamo Drafthouse Cinema that he is rather fond of.
Despite his various failings he remains an outspoken person who isn't afraid to speak his mind even when intelligence, taste, and even basic common sense would suggest otherwise. For this reason alone he remains an interesting landmark on the internet. Sort of like the Corn Palace but with more information wild speculation on who might be Anakin's hairstylist in the next movie.