I saw Indy 4 the day it came out, as a self-gift for my b-day.
Overall, I think I got my $6.50 worth, but there were things that I would've preferred done differently... or in some cases not done at all.
Brief rundown:
What I liked: action, a little less of a humor focus than Crusade (which I didn't
dislike, just didn't float my boat as much), and... well, Indy kicking ass, but not to the total exclusion of considering things instead of just jumping in swinging.
What I didn't like: The villains weren't, except for Madame Icecube (whatever her name, I forget and it doesn't really matter), anyone I could really work up hatred (or anything stronger than apathy, at that) for, the way I could the Nazis in the previous films. I also would've preferred less of the British guy, and none at all of junior, who for a supposedly smart and tough kid didn't seem much of either.
Personally, I would've preferred something set earlier, say during the OSS time mentioned in the post-nuke interrogation scene (and don't get me started on the "invulnerable fridge with inertia dampeners" thing...), though I understand the need for a "fast forward" given that Ford isn't getting any younger.
And of the complaints online I've seen about "letting Lucas run wild with things", no one seems (though I haven't actively sought out online opinions) to mention the obvious Lucas connection: the race at the beginning, which seemed to have absolutely no reason for being there other than to remind people that the guy who gave us "American Graffiti" and the pod race scene from "The Phantom Menace" was involved with Indy 4.
Overall, I'd say "Indiana Jones and the Kingdom of the Crystal Skull" gets about a 7.5 out of 10, with it being an Indy film being worth about 1 point in and of itself.