Excellent story and solid voice-acting. Perhaps a bit too much pixel-hunting.
Its strongest point, by far, is the story, which is complemented by the mostly stellar cast of voice-actors who bring the characters to life (the voice of Arsene Lupin, however, was a bit lacking here). You'll untangle clues and puzzles left by the antagonist, the gentleman thief Arsene Lupin, as he leads you on a chase around London on a mission to humiliate England by stealing its most valuable possessions. The ending is really killer, and may totally take you by surprise.
And now the bad points. First and foremost, pixel-hunting has always been, for me, the most annoying aspect of the adventure-game genre. But what is a slight annoyance in a 2D adventure game becomes astronomically so in a 3D adventure game. There is simply too much wallpaper to paint with your mouse cursor; and the effort can be a bit overwhelming. In addition, some of the puzzles are so maddeningly difficult that, as someone else already said, you would actually have to be Sherlock Holmes to figure it out. Because of these two complaints, I would consider the game unplayable without a walkthrough (unless you possess either great genius or great patience... or an affinity for self-torture).
I could certainly go on about the pros and cons of this game, but I know that no one wants to read twenty pages of my own asinine opinion. Let me just say that it's a great game for fans of the Sherlock Holmes books, for fans of the previous Sherlock Holmes games, and for adventure game fans with a bit of a masochistic streak. Just make sure you have a walkthrough at the ready.