Yes, "Indiana Jones and the Kingdom of the Crystal Skull" is canon. It is an official movie released by George Lucas, directed by Steven Spielberg and starring Harrison Ford in the role of Indiana Jones.
Given Lucas' quote that Rocket Surgeon posted, I don't think there's any question that LucasFilm considers it canon. But as far as the personal view versus official canon, well I guess a fan could discount any production from a franchise that they personally dislike.
In any case, I happen to like KOTCS along with all the other Indy movies. It was awesome seeing an Indiana Jones movie in theaters and I am so glad to have had that experience. (I also recently got to see "Raiders of the Lost Ark" in theaters when it was briefly rereleased to IMAX theaters last year- that was pretty incredible too!!!
Now if I could only get a chance to see TOD and LC in theaters....)
I personally like to include all the movies, the TV series, the novels, comic books and video games and most everything from the expanded universe as part of the canon. (Though some entries may be looser than others- i.e. The Find-Your-Fate books I wouldn't hold to as strongly as MacGregor's, Caidin's, McCoy's, Hohlbein's and Perry's and Windham's and Rinzler's novels. The Young Indy novels I would also hold to more strongly.)