Per an interview with Comic Book Resources, Brian Michael Bendis, who has been writing both All New X-Men and Uncanny X-Men is stepping aside come issue 600 of Uncanny.
So that’s it. I’d like you to open this article by saying the reason I’m leaving the X-Men is Marvel is killing them all, or turning them all into Inhumans, and either I got on board with the program or I was out! So I’m out! Because I WILL NOT ALLOW THE TRAVESTY OF WHAT’S GOING TO HAPPEN TO THE X-MEN DURING “SECRET WARS,” where you find out they were all Magneto’s children the whole time and that Magneto was the first Inhuman! I didn’t want to be part of all it. So I’m excusing myself.
So there you have it! I haven't kept up with his run to the extent that I would like, but what I read of All New X-Men I rather enjoyed, certainly moreso than anything he'd written for Marvel since the days of Daredevil.
Who takes over next is anyone's guess, but all eyes will be on what Marvel does with the X-Men and mutants in general given their current stance on anything Fox has the film rights to.