I'm sold on the Doctor exploring all time and space, but as a plot device there needs to be "fixed time" or else a time traveller could always, always go back and fix everything. A story can't work like that, so I suffer the notion there's "fixed time" in order to avoid endless deus ex machina moments of the Doctor going back and saving the day, repeatedly, yet again.
Having it spelled out as a "fixed time" moment was necessary, so we know it's real and permanent and not just a cliff hanger, with us seeing 'em back to normal in the next episode.
The Weeping Angels are proper scary, I was sighing that they'd been brought back (after last time's weak storyline over 2 whole episodes) but I liked it. Rather a lot. Man, Moffat can write . . .
. . . and as an end for his companions, for them to be happy, together, and share that with him, it was a pleasing resolution to writing them out. Amy's summary of all she did really had me going, too. A moving and well executed farewell.