Monaco: What's Yours is Mine is one of my favorite games. I just want to start with that. I got this game a couple months ago and I'm hooked. Yeah, it's a little frustrating at times, but only in that sort of "Aargh I'm not quite good enough for this level!" way.
Monaco is kind of a fun time-waster singleplayer, but where it really shines is multiplayer. It is FANTASTIC, and not just for the sake of completion. It's so much fun to run through the levels, causing chaos, or try to not get spotted at all. Make sure you discuss your plans with other players, but don't be surprised when things fall apart because of an unlucky corner with five guards behind it.
The way powers help your friends makes the teamwork element of this game happen so naturally you might not even realize you're doing it at first. The hacker runs through a hallway trailing viruses, and the laser grid in your way just dies. The Mole digs a tunnel, and you escape through it to a nearby shrub. And everyone loves a Lookout. If there's one character who doesn't play well with teams, it's the Pickpocket, whose power is purely self-focused and who tends to hoard all the coins. If everyone's okay with that, by all means, play him. But if you're just browsing for random games online, it's polite to not use the pickpocket.
The difficulty wobbles a bit around the start of the second act, but it pretty much goes steadily up from there. And keeps going up. This works to the game's advantage. The difficulty curve gives you enough time to get used to the system, and then slowly works in new elements and characters, and by the time you're dealing with the really hard stuff, you've got at least an idea of what to do. You may not be able to do it on your first try, or even your first ten, but you should be able to understand at least the theory of how the pieces fit together.
Replay value? The game has incredible replay value. Not only are there eight characters to play through with, but there's an entire community of fan-made maps to play, and once you've done all the main story, there's even a zombie mode for more violent fun! I've already beaten the main story, and I'm working my way through the stunningly difficult final campain, "Fin". I am curious how they might expand the story that I thought was over, but it seems to be a well-written wrap-up.
The story is simple, but has a few twists to make it more satisfying. Without spoiling too much, I will say that I would have been satisfied with the game if it had concluded with "Identity". The two (much shorter) extra acts were icing on the cake. It's a pretty standard heist plot, with a little bit of "how we got here" thrown in, betrayals, and other standard tropes of the genre, but it managed to surprise me in places, which made me very happy. It's not often you get to play a mystery from the villain's perspective and still be surprised.
If I'm ambivalent about only one aspect, it would be the artstyle. The sprites are charming, but it's really quite difficut to tell who's who at times, and for colorblind players, this would be completely incomprehensible. Everyone looks like squares, including the enemies. The top-down angle limits the perspective, but the backgrounds do not suffer from the same confusion as the characters. They are gorgeous, elaborate, and the way lighting is used is something I've never seen in another stealth game. In some of the levels, I can even look down through the floor to see bits of the lower areas. The music is amazing, but since the levels can be a bit long, it loops a lot. It's by no means grating, and it spreads the clips out with slow, quiet periods, so it doesn't get stuck in your head too often, but it could have used a little variety. I loved the musical cues for when someone got spotted or an objective was completed, though.
All in all, a fantastic game, and one that I'd readily recommend to anyone who likes stealth games, co-op games, or mysteries.
Singleplayer: 3/5. A little tedious, but fun and challenging.
Multiplayer: 5/5. The game's strongest point. One of the best co-op experiences I've ever had.
Art: 3/5. The sprites were limited, but the settings were gorgeous.
Sound: 4/5. The music was excellent, and I wish there had been more variety to it. The voices of the characters were delightful, even if I couldn't understand a word of french.
Difficulty: 4/5. Quite a challenge, but not so difficult you become disenchanted. You learn and get better noticably over time.
Replay: 5/5. Many levels, community-made maps, and excellent multiplayer. The end is just the beginning.