I liked the episode because I'm a fan of the 40's era and the noir film. I liked the clothes and things like that, but I agree that the content lacked umph. It was all Jimmy's imagination, pretty much, so there was very little basis in the truth of the series, although there were some things that stood out, like Lana wanting out of her marriage, Chloe having a thing for Clark still, and the most obvious, Clark leading a double-life as a justice-seeker.
I think this episode was mainly a way to say farewell to Jimmy and let Olsen Fans get their fix of the comic. But I did like it. (this was my first new episode; I've been watching syndication episodes on ABCFamily and CW from past seasons. But 'Noir' was the first time I got to watch a new episode when it premiered.)