Geek out is only partly true for me too. More than the free aspect, though that was important, it was a way to get schooled back up to where I wanted to be. At one time I was at a higher technical level but that had kept coming down as I got further from hands on technical jobs. I missed getting my hands dirty and needed to get more technical again.
I selected geeking out, but that's only partially true. Unfortunately, 4,000 characters isn't enough of an allowance to list all of the reasons as to why I've been a Linux user since I was a kid and have stuck with it despite the supposed "user friendliness" of other Operating Systems. :P
Well...not exactly the right answer, but the best out of the 3. I've been using Linux since the late 90's and 100% full time since 2001. I did it because it was the best choice at the time. Nothing else was even close to what Linux provides and at this point I continue to use it because it still serves the purpose and the fact that I'm better protected and have better tools helps.
I chose "To Geek Out..." but the real reason is that I had just gotten so fed up with Microsoft. I think Windows 8 is terrible and their continued lack of support for open standards is infuriating. I chose to switch to Linux in order to have a system that I felt belonged to me, and was willing to do the work needed to learn it.
Not exactly my best answer but closest to what I would say. I just prefer Linux over Windows and OS X.