A cool idea that lacks substance and fails to deliver on its potential.
The biggest problem with the game is that it is extremely repetitive. Every mission unfolds in the exact same way. Visit the bureau, climb to a viewpoint, perform menial tasks, head back to the bureau, and kill your target. The worst part is that all of these tasks are so simple and they're not the least bit exciting. I also didn't like the fact that there is no incentive whatsoever to complete more than the necessary investigations before going after your target. You don't really learn anything new by doing all 6 of the tasks. I didn't even realize that you could view the results of your investigations until after I'd performed 3 of the assassinations. Even so, the information you get from looking at the maps aren't all that important at all. You can figure out the path you need to take on your own.
As some people have said, the AI in the game is absolutely brain dead. You can kill a guard only 10 feet away from another guard and he won't have any idea what happened. He'll come over to investigate and not be aware of who did it. Pickpocketing people is a joke as well. Very rarely does one of your victims have any idea what's going on. You can follow him for quite some time with your hand out and the person will be none the wiser.
The combat system is also extremely weak. Once you learn how to counter an attack, that is the only move you'll really need. The guards will almost always block your sword strikes so it's better to either counter their strikes or throw them to the ground and hack them up.
The sci-fi backdrop of the game is also a waste of time and the moments where you control Desmond are absolutely boring. To be honest, I don't know why the sequences aren't just done as cutscenes. There is no reason whatsoever to actually control him yourself. You can only do one required task before the game launches into a speech from one of the scientists. No offense, but being able to walk around in a 10 foot radius while another character is speaking does not give you any additional freedom. I understand that the developers were trying to make the game into a franchise, but the modern day twist should have been left out of the game. They would have been much better served by creating a series by assassinating leaders from different periods in history (one game in the series could be the assassination of Adolf Hitler or some other evil person from a different era). Unless the series is radically changed in the next game, I won't be the least bit interested in playing it.