It launched to the public on March 20th, 2013. There are official apps for Android and iOS. I can't speak for the iOS app as I haven't used it, but the Android app is very nice.
With it being pre-installed on KitKat I just used it without thinking about whether it was "out" there yet. Great service that I use every day. You can even set each note with a reminder time or location that'll notify you (on phone and Wear) to, well, remind you of it. Great for things you remember at work but need for home etc...
