My biological father contacted me when I was in my late 30s. About 5 years ago at this point. I was not sure about meeting him, or even talking on the phone with him, so my husband set up an email account just for us where we could message each other. We ended up writing reams to each other.
I would have to say it gave me a whole new understanding of myself. I discovered my father and I have very similar mental patterns, we make connections the same way. I always felt a bit of a fish out of water in my mother's family, where I grew up. After meeting my father, and hearing his family stories, I realized how much I had inherited from that side of the family, even though I knew little about them. I'm just much more a Celt by nature than an Italian. Many other things as well.
All of that said, my poor father was a deeply troubled person, with 3 broken marriages and 4 estranged children to his credit. My own opinion is that he protected me from the chaos in his life with his absence. He sought me out finally when he had some semblance of stability. Also, i suspect he knew he was near the end. He contacted me 18 months before his death.
As for what to expect, you can get anything. A high school classmate of mine tracked down her birth mother and gushed about what a wonderful person she was. This encouraged her to go find her birth father, who gave her a very different reception. For some, a child they surrendered is someone they always think about, and would love to hear from, and others want to be left alone. The birth father of this classmate refused to even give her medical information. (And I think the whole medical information thing was some self deception on her part. Being rejected in this manner by father figure, who just happens to be your actual father, is a powerful experience. 'But all I wanted was medical information' is a bit disingenuous.
I am writing a book here, but i guess my point here is expect anything. I've written a few hubs about my father, and you can look those up if interested. I got to go to his funeral, and meet his family, which meant alot to me.