Complete waste of time
Let me qualify two things before I support my opinion:
1. I love (and appreciate) "film-as-art" films. IMHO, 2001: A Space Odyssey, Apocalypse Now, Magnolia, Eyes Wide Shut, 8 1/2, and almost any film by Kurosawa or Zhang Yimou are some of my favorite films.
2. I did not finish watching this film...I only made it through about an hour and a half.
My first qualification is written simply to emphasize that I am a fan of film and do not deride something as "boring" simply because i don't understand the deeper meaning of a film. My second qualification is included because I have not finished the film and really shouldn't be offering my opinion on it...but of course, I will.
For any film to be successful, you have to have some type of connection with the character (even if he is evil, despicable, etc.) In M, the main character is a child molester whose pursuit by the law is compelling, but he is an empathetic character (to whatever degree) and this lends weight to the movie. Even if he wasn't, the hunt for him makes an interesting film. Dave Bowman (in 2001) is neither a sympathetic character nor a villain. He is quite emotionless throughout the film but there is still a connection made with the audience due to his predicament and the larger picture Kubrick is painting.
L'Avventura, on the other hand, is simply a picture about annoying, petty people...nothing more. It's arguable if the film is "wonderfully shot" (as I have read)...personally I found it to be adequate...neither wonderful nor pedestrian. The direction is better than average but not mind-blowing either.
So what is left? The story: a disappearance that doesn't get solved. That's fine by me if Antonioni has a higher purpose than simply depicting a mystery. Apparently he does, from what I've read. To sum it up succinctly: money can't buy you love and the decline of morals in 1960's society.
Is this message potrayed adequately? IMHO, not at all. In order to see this point, one must have some empathy towards the characters. Unfortunately, every single character in this film is a shallow, self-absorbed, a**hole (for lack of a more appropriate term). If they are meant to represent the lack of morals in 1960's society, they do a grand job. Does this make the film entertaining, thought-provoking, deep, and/or compelling? Absolutely not. I've never cared less about characters and what happens to them then I did in this film.
Very often films require multiple viewings to fully understand the whole picture being painted. Just as often films are devoid of anything interesting or entertaining on any level. This film falls into that latter category.
ADDED: Just wanted to add that now that I've seen La Dolce Vita (a film about similar topics) that L'Avventura appears even worse than before. La Dolce Vita is a far more interesting, artistic, and appealling take on similar subject matter (though it is a bit long).