I'm really stoked for Reign of Winter, and I'm thinking of buying a subscription to make sure that I have easy access to the books and the PDFs. But I'm wondering whether it would be better to wait until I can begin my subscription with Reign of Winter, or of it's worthwhile to pick up the last two volumes of Shattered Star (and maybe the earlier volumes).
Currently my group is in the first "book" of the anniversary edition of Rise of the Runelords. I'm aware that Shattered Star is a "sequel", but would you really recommend it? Or am I better off waiting?
If you're playing in RotRL, I'd advise against purchasing the last two volumes of ShS. You should start with the first volume of RoW.
If you're not playing RotRL, then you could start your subscription as soon as possible. The APs are full of goodness for any DM (bestiary entries, background articles, etc.)