After a few days thought... Im not a fan of the ending, but it could be a lot worse... most of the time a finale doesnt come close to expectations and could even ruin a series (see Sopranos, The). I think the writers needed some sort of major event and/or major twist at the end. The whole "purgatory" thing was just fabricated from material introduced in the this season.
Also, if you look at the final conflict it is pretty unsatisfying. Like WTF was the point of Ben turning in the 2nd last episode? The final fight wasnt good at all, there were much better fights throughout the series. The final resolution to the whole thing was unplug a mysterious plug so light stops shining in a cave, throw the evil guy onto rocks, then plug the thing back in?
I saw much better conclusions written by fans on the internet over the course of the past few years. Pretty bad writing here iMO. The series (potentially one of the greatest serieses ever IMO) went out with a whimper. Also take into account that fans have been looking forward to a huge "mind f*ck" at the end for 6 years.