It looks like they spent a fortune on that episode. The aliens were pretty well realised and there were a lot of them.
It had a lot of interesting ideas, even if the weren't wholly successful. I can see this dividing opinion, I give them full marks for trying something different but take one back for the resolution.
Good stuff with him sneaking back in time to check out Clara's past to try and figure her out and her starting to twig to it.