Discworld Noir is the one true masterpiece that rises above all other adventure games.
The story is truly enthralling and brilliantly funny. The characters are interesting and they are alive, much more than just means to an end. The humour is great, intentionally filled with wonderful noir clichés and it works well even without previous Discworld knowledge . (I had only played the previous games and hadn't read a single novel before first playing this game through. After this game I did start reading a bit though and I do think the are some inside jokes that opened up to me only at later playthroughs.)
There has been some criticism about the graphics but I think they work just fine, by no means they can be called bad. In my opinion the style suits the game very well.
The one thing Noir has been praised about is the logic. And I agree, this game is the most logical adventure game I've ever had the pleasure to play. The puzzles are also just the right difficulty and they are actually fun to solve. The notebook interface works like a charm and I am surprised to see how little it is used in other games. The whole game flows smoothly and while you really want to get to the finish you are also hoping it would never really end.
I also was impressed how the scents were implemented. If you haven't played the game yet this might seem like an odd sentence but you'll understand, I'd better not spoil. I am truly envious of people who haven't yet played this game and will have the pleasure of doing it the first time. I think I'll be installing this game soon again. This was one of the few games I bought at full price right when it was released and I couldn't be happier about it.