Endings. Endings are important. That's pretty undeniable. A good ending can make you forgive a lot about a mediocre everything else, while a bad ending ensures that you leave talking about the worst parts.
Endings are also quite difficult. Look at Iron Man - a great movie through and through, hurt by a terrible ending. If not for the Nick Fury scene after the credits, fan word of mouth likely would have been much worse than it was. Indiana Jones and the Kingdom of the Crystal Skull was another movie with a terrible ending, only they didn't have a massive fan-wank scene to follow it up, so when I left the theater, I heard a whole lot of apathy.
People enjoyed the movie. I know this because I was there. The first third of the movie was filled with laughs and thrills - insane action set-pieces falling pretty naturally into even crazier action set-pieces, punctuated by wit and impressive visuals. Things were going strong, and they moved onto the grave-robbing and insane archaeological mysteries, and things kept going strong. It got creepy, but the movie never lost its sense of fun, and once the KGB found them again, things got even crazier, and the action kicked it up another notch.
Things only really got bad once the 'mystery' was solved and the Skull returned. Then, it just became bad. The Ox became an exposition machine, spouting off unimportant things that the writer just seemed to think would be SOOOOOO super cool (Interdimensional, point in fact) that didn't effect the actual story, except to make it seem overly pretentious and kind of...silly. And while an Indiana Jones movie works fine with silly - thrives on it, in a way - pretentious is fatal to it.
The movie isn't a failure. It's pretty good, actually. There are greasers, jocks, the KGB, Cate Blanchett as a sexy Russian psychic warrior spy...thing. It's not exactly believable, but it's a movie, and it's fun as hell, but the ending kills the goodwill, and any momentum it had. It hits a brick wall of bad dialogue and unexciting visuals, of anti-climax and then ends with a wedding.
Is it worth seeing? Sure. If you like Indy movies, you'll like this one. Reviewing this movie is a little weird. If you want my advice, just leave as soon as they get down into the treasure room. But, you know what, you won't. And you'll be disappointed. And you'll remember what I said.
And you'll curse me.