Just finished watching the finale. I'd say it's relatively solid, if you just look at its parts rather than its whole. Anybody that wanted to see Martha come into her own should certainly get a kick out of it. Anybody that enjoyed the bizarro evil anti-Doctor whimsy of The Master will find more to like.
Is the "solution" to the crisis a stupid, unexplainable deus ex machina? Yes, but it's not like there aren't 101 other episodes where something dumb resolved the conflict.
I'd be curious to know what the main complaint about the episode is......the only major problem I can see is that The Doctor is out of commission for about 3/4 of the show, but that's remedied by the fact that The Master takes his place.