In a cute message to celebrate their 10th wedding anniversary, the actor gave his score in a photo posted to Instagram.

"A perfect 10. Happy Anniversary, Mrs. Downey," the caption said.

Downey Jr is not the only one who thinks it is a perfect pairing.

Sherlock Holmes director Guy Ritchie has called it the most "perfectly symbiotic" marriage in Hollywood.

"The reason I call them perfectly symbiotic, it's that they each represent the other side of each other's coin," Ritchie has said. "There's no clashing over the same space, although they're always looking in the same direction. It's a very rare thing they have."

Downey Jr, 50, met his future wife, 41-year-old Susan, on the set of Gothika in 2003.

It wasn't love at first sight for Susan, who was producing the film. In fact, she found him "interesting but weird".

"We were up in Montreal prepping for Gothika, and we had lunch with the director and Halle Berry," Susan recalled of their first encounter.

"Everybody else ordered Japanese, but Robert told us how oatmeal was the 'superfood'. He brought his own packets of oatmeal to have at lunch. And he had this box of various herbs and stuff. And then he started doing these yoga moves. I mean, he was interesting but weird."

The Iron Man actor was interesting and very interested and had to ask her out three times before she said yes.

Within three months, he proposed marriage, despite the fact that he wasn't yet divorced from model-singer Deborah Falconer, whom he wed in 1992 and with whom he has a son, Indigo, 21.

Eventually Susan relented but insisted on a two-year engagement.

The couple married in August 2005, in the Hamptons.

They have two children, son, Exton, 3, and one-year-old daughter Avri.

"I was very focused, driven, rigid, work-oriented," Susan has said of her life before Downey. "I didn't care about having a family or making a home. I didn't think about kids. It's not that I didn't want those things, I just didn't think about them. And then I had someone who came in as a tornado, this creative, beautiful ball of insane energy and passion. And it completely opened me up."

Downey Jr credits their relationship with helping him to overcome a decades-long substance abuse problem.

He has called their time together his "Great Transition". "She is the font of all good things," he says.

Fairfax Media