Ranks third out of the four Indy movies (behind Raiders and Last Crusade). The villains are less effective, and though Ford and Shia have pretty good chemistry (not to mention the show-stealing Karen Allen), they're never at the level of Ford and Allen in "Raiders," or Ford and Connery in "Crusade." They make their way through a variety of exciting obstacles, but it feels like something's missing to tie it all together.
I always preferred when the mystical aspects of the adventure were left until the object in question was discovered (e.g. the Ark, the Grail) so we could focus on the adventure. A lot of this movie slows down for exposition and the pacing seemed a bit off at times.
That said, the acting was great, the effects were good as usual, and if all you're looking for is an Indy film, there isn't much more to look for. It'd be nice to have a tighter, more effective screenplay, but this IS a George Lucas film...