I played a few hours of it, so I'll post a quick summary of my initial impressions.
Gameplay: The controls are horrible, and the shooting mechanics are average; running is awful, camera is annoying, and aiming sucks. I can't find a way to turn invert aim on or off.
The detective stuff, on the other hand, is pretty fun. Basically, you investigate the area looking for clues like in Phoenix Wright. Also, there might be a few puzzles or action scenes after investigating the crime scene. I like that you can progess without finding all the evidence, and that you can turn off evidence hints off. It makes investigating the crime harder and more authentic. I want to put my mind into this game. I want to be a real detective and not be babied. I will put every criminal behind bars with my superior intellect.
Interviewing suspects and witnesses is well-done, imo. Basically, you ask a question regarding the crime, and after they answer, you can accept their answer as truth, doubt it or call a bluff. If you call their answer a lie, you have to provide them with the appropriate evidence to support your claim. If you pick the wrong evidence, you get sort of penalized. You might miss some crucial information, or they might refuse to talk to you after that. Also, the less questions that were successfully answered, the more likely you'll fail this case.
Presentation: One word: unparalleled! The facial animations are fantastic, and you have to keep an eye on the guy's face you're interviewing to find out if he's bluffing or not. Brilliant stuff here, imo. Music is pretty good, voice-acting is top-notch, the setting is very interesting, and the city at night looks wonderful.
Story/characters: The characters are likable including Phelps, the character you control. It's kind of funny how serious Phelps takes his job, but then again, I'm like that :). The cases have been interesting to say the least, and I read they're based on real-life crimes, which is intriguing.
I'm liking it so far. It's way better than that borefest RDR. One major concern I have with the game is the replay value; replaying the game won't be as fun as the first playthrough since I'll know who did it, what clues to look for, where to search, etc.