Flawed but fun
I just got this game recentlly, having ignored it for a long time due to poor reviews. I've been very pleasantly surprised.
First, it's just drop-dead gorgeous. So gorgeous my wife acutally asked me about it: "what was that amazing looking game you were playing?"
Second, when everything comes together, it's just awesome to play. Frantically running from the guards, bounding across the rooftops, quietly assasinating someone on the street and calmly walking away...perfect.
Sadly, it doesn't always come together. As many other reviewers have mentioned, it gets very, very repetitive. Ride your horse to the city. Go to the guild. Climb all the towers. Do all the investigations. Do the assasination. Return to guild. Rinse, repeat. Though the individual bits are still fun, it can be a little difficult to bring yourself to do it all over again. The repetitiveness occurs in the voice acting too -- the dozenth time you hear a saved citizen say the exact same thing is a little tiresome.
Flaws aside, it's worth playing if you have any love of stealth-style games.