I know they had to make it under certain constraints, but I didn't like it when those constraints became too obvious within the film, I very much didn't care for the way the Shadows were arguably refactored, and I found the ending intriguing but also a bit WTF in terms of the impact to the larger timeline that would ensue. IIRC there were also times when the humor probably should have been dialed back a bit.
I did, overall, find it very watchable...but not watchable enough that I've watched it more than the two times (one normal, one with commentary). It feels more like a pretty good S5 episode than a gripping S3 or S4 episode, I suppose you could say.