Hi
Sorry I'm so late to join this conversation. I came the long way round.
What happened is that I'm not really a TV watcher any more and didn't watch this series at all (even though I'd heard it was a good one). But then I noticed that River had been in the Christmas special, so I wanted to catch that one. And I figured I had better watch the series finale first just to get the gist of what's been going on. And then when I saw the teaser to Heaven Sent, I quickly realised I needed to also watch Face the Raven! So I have ended up watching the last four episodes back to back over the last two days!
Well I'm glad I bothered to watch them. Heaven Sent was especially good, and the others were entertaining. I thought The Husbands Of River Song was a really good Christmas episode and Mr Moffat has still got the 'old magic' as far as I'm concerned.

I will always be a fan of that man, even though I don't really watch TV any more.
So anyway, I just wanted to say something here about this episode, something which has no doubt been said a lot already (I haven't read the thread) but I still feel compelled to say it…
While the Doctor was technically in the prison for 4.5 billion years, he was not consciously aware of those 4.5 billion years. As far as he is aware, he was teleported from London 4.5 million years into the future, stepping out of the teleporter for the very last cycle. From his point of view, he spent only maybe a few days in there, and he didn't even have to go through being killed and crawling up the stairs. There may have been billions of cycles, but none of them were aware of the memories of the others.
So, in one sense, the Doctor could rightly tell Clara that he spent 4.5 billion years saving her. But technically he didn't have to go through all that time himself, so he was being a bit dishonest there. He spent days in that prison, not millennia.
I know this point may seem trivial, but 4.5 billion years is such a ridiculous amount of time that this really is quite a big deal with huge implications. If, for one second, we were to believe that the Doctor was somehow conscious throughout the entire 4.5 billion years, then I would have to say that this would be just too ridiculous a premise. Just too unthinkable. Rory guarding the Pandorica for 2,000 years was pushing beleivability to its limits, but we (just about somehow) managed to swallow that. But 4.5 billion years is unthinkable.
I also don't understand about why some elements of the rooms were not reset (like the painting, and the clothes hung up, and 'BIRD' scrawled on the floor). But I can't really be bothered reading through the discussions to see speculation on this!