Yes, it is in stores. At least, my 'local' store, Games Plus, has it in stock. I'm till getting my feet wet with it so don't have much to say. Unfortunately, it looks pretty good and I say unfortunately, because I've been using Cry Havoc to experiement with and enjoy it. To have this also be good is going to cause some issues with which one do I use.
I'm very curious to see what happens in mass combat systems in 3rd party books as well since much of this book is Open Game Content providing lots of options for those publishers who want to go that rotue, they now have some very tough decesions to make.