Angry Birds Stella is out right now on the App Store. And the Google Play Store. It doesn't seem to be out on any of the Windows platforms it's supposed to be coming out on though, so that's a bit of a shame.
The game is basically Angry Birds, but with a few tweaks here and there and a shiny coat of polish lavished atop it. The birds do different things, there's some free to play trappings, and it's all sort of the same as it's always been.
At review we gave the game 6/10 and said "it's Angry Birds. If you care about that any more then you'll probably find something to enjoy. Just watch out for the timers."