My husband and I met when we were in our mid-30's. We met on Match.com, actually. Our first few phone conversations were 4+ hours. Our first date was at Busch Gardens. It was almost an instant thing for us and we attribute that to two things: 1) having already 'been there, done that' in terms of relationships, and 2) we truly are a perfect match.
For the most part, I'd say that we compliment each other, and yet we often share the same brain. We bought each other the same exact gift for our first valentine's day. We complete each others sentances and often communicate without speaking. We often get asked how long we've known each other and most people can't believe it's only been 7 years now (we've been married for 6 of them). And yet, there are sizable differences. I'm controlling, he's not. He's quick to anger, I have more patience. We are both engineers, so we are both analytical and practical. We have different views about raising children, but since we've chosen not to have any, it's not really an issue. We value the same things and share the same dreams...so it all works.
Until I met him, I didn't understand those people who had to call their significant others every chance they got. I didn't understand how one could want to spend that much time with any one person. I was fiercely independent, and I expected to always be that way. I am. But now I have someone with whom I want to share that independence. Weird? Maybe, but it works...for both of us.