I enjoyed it, but I'm not sure that it ever became totally clear to me what was going on. This isn't unusual for me with programmes that Steven Moffat writes or co-writes. I sometimes have the uncomfortable feeling that senility may be setting in earlier than it should!
It's not senility: it's Moffat being careless. His tendency to dangle things in front of the audience and then never properly resolve them is deeply annoying. He seems never to have heard of the Law of Chekov's Gun.