I've been using it since July 2013. I'm very, very pleased with it. I can't imagine not using it anymore - on my Nexus 7, my laptop, my main desktop machine, even my NAS. It's just the thing for sharing files with friends.
My only misgivings, as many have stated, the closed-specs. I keep an eye out for alternatives, but so far nothing else.
I just figured out a very handy use for BTSync: for monitoring and updating your website. Bonus: if your site gets pwned, you'll see the sub/sub/sub/sub/sub/sub/sub/fake_bank.html on your dev machine...so you can say "whaaaa!?" and kill it.