I used to think I wanted to write mysteries, but once I discovered how challenging it easy to make that formula new and fresh, I changed my mind. However, I do end up putting some kind of mystery in most of my books, but it's usually a secondary plot. Many books have mysteries in them like that -- it's part of what keeps us reading. We want to discover the unknown.
As a kid, I read probably 80-90% mysteries (Phyllis Whitney, Alfred Hitchcock and the Three Investigators, the Happy Hollisters, etc). I moved on to Agatha Christie around 12 or 13, and then Mary Higgins Clark as a freshman in college. I wrote my senior (high school) paper on Christie, and when I did that, I realized I'd read every one of her books -- except for the Miss Marple series (because I never did like Miss Marple much). ETA: Of course, as Joan reminded me, I also devoured all of Mary Stewart's books in junior high and high school (and still reread them regularly today).
Nowadays, I don't read many new mysteries. There aren't many for teens, actually. I've read some Gail Giles and a couple of others. They've been fine, but since I like Linda Howard (romantic suspense) -- and because mysteries are overly familiar to me -- it's hard to find YA books which live up to that. I think my favorite right now is Kim Derting's books (The Body Finder). I just finished Rosebush, by Michele Jaffe, and that was also good (thanks for the rec, deenaml), although one way she challenged the reader to find the 'bad guy' was to make everybody kind of insane ;)
How about you? What do kinds of mysteries do you like and/or read/write?