Gillen's run was incredible considering that the clock was ticking on him from the moment he started. He was in the unusual position of being on an X-flagship for the semi-long-term but still knowing he was simply filling time between status quos.
Did we get the full-on Gillen insanity we could have had if he wasn't maneuvering Cyclops into checkmate while navigating a pair of line-wide events and an X x-over? No. But, did we get a handful of issues that were the best UXM has been in decades and the first good writing on the Rasputins in just as long? Did we get an Emma that finally felt arch again even as we got rationalization for Cyclops's seeming descent into tyranny? Yes!
Because of his foreshortened run, I would place Gillen behind the long-running heavy-weights - namely, Claremont, PAD, and Carey, and the self-contained stories like Whedon and Remender. However, if we're going sheerly on characterizations, pushing his artists to deliver their best work in years, and his ability to send fans into a tizzy month after month, at the moment I grade him just a notch below the Claremont/Byrne era.