In short this is a well acted story that must be experienced, but with lame gameplay it is not enough of a game to warrant a high price.
The story is pretty much perfect, the acting is perfect, the visuals are excellent.
What breaks it for me is the fact that the core gameplay is an interactive film. The game is like having a friend with you that has already seen this film and even if you ask him not to spoil it, he will gleefully give you tiny hints through his body language. I'm not saying the game plays itself but if you have doubts this game idea can really be made into interesting gameplay, it is probably as bad as you fear. Walk around the crime scene, look for tiny objects and tracks and have your character comment on them, occasionally solve simple puzzles, interview people that for the most part know nothing about subtlelty (Some will surprise you, but most are overplayed and obvious).
Occasionally, or actually rather frequently, you must engage in pursuits and shoot outs which is like a stripped down GTA, very bog standard gameplay mechanics but with rather jagged presentation, there is no smooth transition between investigation and action, it's like suddenly being thrown into a whole other game once the gun is drawn. The amount of bad guys you kill (to no consequence) and the amount of cars you completely demolish is bizarre, not at all very noire. The car chases are probably the weakest part of the game, the driving is not fun and flipping over the bad guy's car over and seeing all the wheels, doors and windows pop off with poor physics leaves a really bad taste in your mouth. Where is the subtlelty? This feels like it belongs in a much older game, not a mature cop noire story.
The saving grace for this game is that the characters are excellent. The writing is the best of the best, the story has it's surprises and unusual character dynamics (especially the main character is an interesting do gooder who comes into conflict with pretty much everyone in the game in some way). The ending is a little rushed and over the top, but excellent and natural feeling dialogue makes it work. It took me about 20 hours to get through my first playthrough and I completely botched a few cases, the interview mechanics can be a little confusing the first time around (especially how to accuse someone of a lie) and there are a couple of cases that feel very counter-intuitive to me. The overall experience however is awesome and unique if you can forgive the game for it's damning short comings in gameplay.