I don't know them personally, but through interviews and observations, they are the best of friends. They were catapulted to success during the same time and conceived once of the greatest franchises the world has seen (Indiana Jones) while they were on vacation in Hawaii together.
Sadly, like most friendships, objective viewpoints of the other are often seen with a blind eye. Hence, they gave the world likely the most disappointing sequel of all time (Indiana Jones and the Kingdom of the Crystal Skull... and no, Star Wars The Phantom Menace was not a sequel, otherwise it would be #1). Spielberg has gone on record saying that he wasn't that thrilled with the McGuffin of the fourth film. It is also know that Frankl Darabont's draft of the sequel was something Harrison Ford and Steven Spielberg loved. But Lucas rejected it.
"It was a tremendous disappointment and a waste of a year," Darabont later told MTV. "I spent a year of very determined effort on something I was very excited about, working very closely with Steven Spielberg and coming up with a result that I, and he, felt was terrific. He wanted to direct it as his next movie, and then suddenly the whole thing goes down in flames because George Lucas doesn't like the script." A despairing Darabont confronted Lucas directly. "I told him he was crazy. I said, 'You have a fantastic script. I think you're insane, George.' You can say things like that to George, and he doesn't even blink. He's one of the most stubborn men I know."
Spielberg has often also stated that in the end, Lucas is the producer. They often succumb to each other's will. Although Spielberg admitted that the Nuke Fridge scene was his own idea (sigh).
Tangent: Darabont's script was called Indiana Jones and the City of the Gods. While there are many similarities, sequences, and even story elements that can be found in the eventual Crystal Skull movie, the draft is much better written, void of the utter silliness of the first. And there are no gophers or Mutt Williams (Shia Labeouf).
You can read the leaked script here...More questions on Movie Business: