I have been a member of Sherlock Holmes societies, and of the International Arthurian Society (which is academic - not people dressing up as Guinevere and saying they were Merlin in a former life) in the past. Not currently, but I am still interested in both areas.
I studied the Arthurian legend extensively in university, after being at least somewhat interested in it since a young age, and went on collecting books for years, and had a short story published.
Sherlock Holmes is one of my particularly long-standing and beloved obsessions. I first read the stories starting from about age 7. I was especially obsessed with the TV series starring Jeremy Brett when I was in my teens. I've had a few Sherlock-related things published and though I don't read every single book related to him any more (ie. new stories written by people other than Conan Doyle), I still read and follow a lot of it. I love the new "modern" series with Benedict Cumberbatch, really great stuff, though I wasn't sure I would like it.