Okay, I've never been a huge fanboy of the series, but I enjoyed the previous Indiana Jones movies. I have no problem summoning the level of suspension of disbelief required to enjoy watching a guy swing around on a whip, evading Nazis, and even recovering supernatural artifacts that occasionally melt the faces off the aforementioned Nazis.
Unfortunately, the stunts and chase scenes in "Indiana Jones and the Kingdom of the Crystal Skull" are so completely unbelievable and obviously CGI-based that it completely takes the viewer out of the movie. The dialogue -- never a strong point of Indy films anyway -- is laughably bad at times, especially early on in the movie. Even Harrison Ford's natural charisma can't save most of the awful one-liners he's spouting.
The story itself has potential, but you can see most of the "twists" coming from miles away, and there are several scenes where it seems obvious that the producers added plot points to complement the stunt sequences, rather than the other way around. Need another escape scene? Let's just have Indy get captured for the fifth time. Fight scene? Let's have these weird half-human natives jump out of the walls with absolutely no explanation given for their existence (and nobody in the film has the presence of mind to be particularly surprised about the attack).
Honestly, if I'd been in the theater alone rather than with a bunch of friends, I would have walked out about twenty minutes in. Even the ending made me roll my eyes, but I won't spoil it for the rest of you who haven't learned the "don't give any more money to George Lucas" lesson yet (damn it, I knew better!).
It's a rare movie that actually makes me feel embarrassed to be sitting in the theater watching it, but that's the one stunt "Crystal Skull" executes flawlessly.