Never had a successful relationship where the person was not at least on the same level of me in intelligence...but the intelligence was not necessarily the same. While I might have logical and linguistic intelligence, my partner has common sense and musical intelligence. Have you read Howard Gardener's thesis on the 7 kinds of intelligence? Mark Vital expands the number to 9 kinds. It is interesting and shows that some combinations do work well together while some do not.

There is such a thing as emotional intelligence also. I know some otherwise very intelligent, well off people who can't empathize or emote themselves out of a paper bag.
My late dh had the intellectual variety off the chart. Our relationship, while often a power struggle(two hothouse flowers), lasted 20 years.
I miss being able to ask him questions about all sorts of things. "Where did you turn the water off in the basement when we'd go on trips?" "Where did you put the title to the truck? Why isn't it in one of the safes?"
"Do you still want me to read The Holographic Universe so we can discuss it?" ?

Have been "seeing-ish" someone who is very mechanically intelligent & semi emotionally.
I'm still a free agent.

Someone more intelligent. It was great. It fell apart... For reasons.

My ex girlfriend is a lot smarter than I am--she's an astrophysics student.

However, it wasn't the intelligence difference that killed the relationship.