I love Goodreads, and have been on the site since.... well, I can't remember actually - 2010 I think? As already mentioned, it's a great way to keep track of the books I've read, look up reading order in a series, get reviews from people you trust before deciding to buy a book, and I can't even begin to count how many free books I've gotten thanks to information from groups I've joined.
It was joining my first group in 2012 that really made Goodreads fun for me. I've been able to beta read books for several authors, or get free advanced review copies, and just chat with like minded fellow readers.
However, there is one somewhat unique reason Goodreads will forever be regarded fondly - I'm here in the UK, and married to my Brit, because we met on Goodreads. In fact, it was a moderator of one of the groups we were both members of who recommended UK-Yankee when we had no idea how to start the process of living in the same country.....
And the rest, as they say, is history!