I am big Goodreads fan, both as a reader and as an author.
As a reader I use it to keep track of my books. I also use it as a place to talk books with other readers. It's fun to get into a good discussion of a book you enjoyed/loathed with other readers.
I also use it to find reccs for books. I follow the reviews of a few people who have taste I trust, and get little notes to tell me when they've reviewed a new book.
As a book reviewer I like it because it gives me a bigger audience than my blog alone.
As a writer, I find being able to connect with readers and other writers invaluable. I like the fact that I can sell my book through them. I like the fact that they connect into Amazon as well.
The sheer size of the thing is daunting. There are probably tens of thousands of groups and that can make things confusing. I went for ones that covered things I wanted to talk about/were related to what I write about (usually the same thing) and got to know people.
All in all, I'd say it's worth the time to get to know.