I managed to get my copy in the post earlier today and have played a pretty huge chunk of it. I'd say going by achievements that I'm coming up on the halfway point and I have been enjoying it, but not without more issues than I would have liked.
I have found the game to be full of some really messed up glitchiness, people disappearing, fights ending before all soldiers are dead, totally unresponsive guards whilst you murder all of their friends around them, some really bad framerate in some of the more action orientated set pieces. Just a really high volume of weirdness, nothing game breaking, but definitely immersion breaking.
I have found the games story so far to be enjoyable although it's nothing you haven't seen elsewhere in the series already. I have played as Desmond and I really enjoyed his first mission, I don't really think this is a spoiler for anyone.
I have a lot to say about the "extended prologue" (late title card) and can't wait to discuss the "Holy fuck!" moment that occurs at the end of it. There has been a sequence that is by far the weirdest thing in any of the games and I'm expecting far more of that.
I have done the first Naval battle, it was far more exciting than I thought it would be.
I don't really like what they've done with the shops, it seems needlessly complicated, I preferred the way it was handled in the last three games, but it's hardly an important part of the game. there's a lot of animals, and there's a lot of opportunities for animal murder. (I stealth killed a raccoon)
Overall it feels like more of the same so far, probably a little more ambitious in scope, there's some awful looking scenery in some of the woodland areas, something I really, really wasn't expecting and some of the British voices are so, so, so, so terrible. Connor's voice is great and so are a couple of other major characters, but some of the supporting cast sound like they're just reading it off of the page which is weird for such a high profile game.
I think the faces look great, there's been some exciting stuff happen, although I was really put off by the three or so missions at the start of the game that had instant fail states, but as soon as it opened up it felt a lot more balanced. I'm also not a fan of the way you can be attacked by up to four guards all at the same time, it feels like a cheap way to up the difficulty as most of the combat is pretty easy.
At the minute it feels like more of the same, I'm hoping for some truly mad/ great stuff to happen. I'd love another AC2 ending moment, but who knows how it'll end. It seems like a good game at the minute, nothing has elevated it to great yet, and the technical stuff is really bumming me out at times.
edit - It also features possibly the best instance of Nolan North swearing.
EDIT 2 -Finished it. Connor's story gets pretty exciting towards the end, but ultimately falls flat. Desmond's unfortunately falls even flatter, which is so depressing as his final gameplay sequence shows a hell of a lot of promise.
The end isn't Mass effect levels of bad, but it glosses over a lot and is pretty much a non-end point for the series (surprise) Very few answers despite the feeling that they were going to lay out all the space people stuff clearly. It's not a clever or thought provoking ending, just brief and a little insulting to people that have stuck with the series for five games.
I am left pretty disappointed overall. I expected more from the storytelling, but all I really got was uneven gameplay. I like the game for the most part, but it's pretty hard to recommend to anyone else over the better (Ezio) games in the series.