I absolutely loved this. I enjoyed The Day of the Doctor, but this to me was the true anniversary special. I can't say much that isn't just a fan-girl fangirling, so I'll get to my one critque:
I laughed when Matt Smith popped up. I understand the poetic symbolism, but most of the poetic symbolism didn't violate my suspension of disbelief. The Exterminate/Kennedy scene was a powerful use of this kind of cinematic poetry. I know that he wasn't literally reading the script at the same time as JFK getting shot, but that's fine, because he doesn't know that JFK is getting shot either -- he doesn't have access to knowledge outside of the in-universe world. Having Hartnall's actor see Matt Smith was a visual symbol that Hartnal couldn't conjure up in-universe -- even if he envisioned other doctors, would any of them be in their mid 20's? -- and so the story is broken and I can see the man behind the curtain winking at me.
I actually think the simplest solution would be to have him look off into the distance, and not let us see what he sees. Perhaps even give us a view of the other characters watching him look off into the future, so that we can share in a mutual "What kind of future must Hartnall have imagined?" moment. That amount of 'twinkle' would be hard for an actor to portray but the guy playing Hartnall could have pulled it off; The amount of which he communicated to the audience was outstanding.
So yeah. Loved. I'd give it a perfect score, even though I recognize that it wasn't perfect. I'm amazed at how captivating it was -- I'd love to see more of those films by Gatiss documenting the entire tenure of Doctor Who. Imagine Colin Baker's movie! Hell of a story there.