I always dread when this film gets brought up online because of the baggage that has been placed on it because of its reputation as "Greatest Film Of All-Time". I think many people take that into the film and approach it with an attitude of "PROVE IT!". No film can really stand up to that scrutiny and without fail there are always those who dismiss the film as "overrated" or even worse "boring".
Overrated is a flimsy critique because it isn't really a critique of the film it is a critique of how other people feel about it. And I have a hard time understanding how anyone can be bored by Kane, it is just so thoroughly entertaining. Some classic films (even films I love dearly) can feel like eating you're vegetables but this isn't one of those films. While it is very much a great artistic achievement at the end of the day Welles was a showman (remember he dabbled in Magic) and he wanted to reach the masses. This is a film that is aiming for a mainstream audience.
I love, love, love this film. Is it the greatest of all time? No because I don't feel there can be any greatest of all-time. But if you insist on naming one you can't do any better than Citizen Kane.
Edit: One more thing...
If you want to check out a really great Marion Davies film that shows off how talented of a comedic actress she was I highly recommend "The Patsy". A fantastic film.