I read a book that Jerry Lewis wrote talking about his relationship with Dean Martin. In it, he details how they met, and how Martin essentially rode his coattail's to success. He also asserts that Dean Martin wasn't a particularly good singer, and didn't enjoy much public success in either his acting or singing career.
He then goes on to talk about an album that he recorded to demonstrate he was the better performer that outsold any of Dean Martin's efforts. I don't think I ever heard it, but I own pretty much anything Dean Martin ever recorded. I always liked the way he sang, and the whole lounge singer, 'Rat Pack' vibe.
He ended the book by saying that he regretted them drifting apart, and that Dean was always welcome to come back.