I managed to get out and see "Indiana Jones and the Kingdom of the Crystal Skull" yesterday, and while I thought it was better than "Temple of Doom," it wasn't better by much.
To be fair, it isn't the fault of the actors. All of the primary stars gave stellar performances. Even Cate Blanchett, who is reduced to playing Generic Russian Bitch #12 due to the storyline, is able to pull off a decent performance. The special effects were decent, there are some nice miniature sections, and the script stayed true to the characters.
The main problem is the myth they decided to tackle. Movies like this work when the back story is well established and generally integrated into our communal consciousness. "Raiders" and "Last Crusade" work because they ask us to imagine for a moment that the legends behind Judaism and Christianity respectively are literally true. While the legends that "Crystal Skull" ask us about are less than sixty years old for the most part, they're still enough out there that we can start to see it. "Mola Ram" was just pulled out of Lucas' ass.
It's still worth seeing just for some real closure to the series, but as long as you go in expecting a big screen version of a TV episode of the Indiana Jones chronicles, you won't be disappointed.