Just finished this last night - easily my favorite in the series.
For me, it goes like this ACR >>>> ACB >>>> AC2 >>>> AC1. Each game has really outdone the previous (which has surprised me, because based on reviews, I always go into the games expecting to be disappointed).
I thought the narrative of this game (Ezio's story) was easily the tightest, most cohesive of the series. I've always found the AC stories to be schizophrenic and disjointed - but Revelations was a solid, engaging thread. Well done. The pacing was so much better than previous games.
And excellent ending - one of my favorite game endings ever, I was really moved. I've always seen Ezio as a wasted opportunity (solid character, but the stories have always been all over the place). But here they nailed the build up, and the ending was really satisfying.
It wasn't perfect - the Altair sections were a little forced (though the ending was completely awesome), the tower defense stuff seemed wildly out of place (though I only did the first one), and the city (though gorgeous) was pretty lifeless (you essentially deal with 3 or 4 characters throughout the whole game). That said, it was a compelling experience that I really enjoyed.
I do think there are some things that need to change for future games:
1) The controls. The whole control system is just overly convoluted. Why am I holding Right Trigger all of the time? I think that a change to something more like the Prince of Persia control scheme would go a long way. The free-running stuff is cool, but I still find it difficult to do some of the (totally awesome, btw) indoor missions (finding the artifacts, etc). I haven't really thought this through (there must be some reason why they use this scheme), but there were a number of times in Revelations where I couldn't help but notice the crazy complicated controls.
2) Overpowered Ezio. It really has gotten out of hand with Ezio's fighting abilities - other posters have said the same thing, and I agree wholeheartedly. I could literally kill every bad guy in the city, and that makes a lot of the story elements seem contrived.
3) Simplify everything. I feel like AC3 will be a good opportunity for Ubisoft to really go back to the drawing board for a lot of the game mechanics. There's just so much... stuff in AC:B and AC:R that they feel a little unfocused.
Bring on AC3!