I'm going to preface what I'm going to say with a few notes. First, I'm only in the middle of 5 and I think it is good. I was also a HUGE fan of the book long before there were whispers of a TV show.
Having said that, I feel like there is a lot of wasted potential. As others and reviewers said, having Purple Man as the driving storyline is disappointing. Even in the book, while it was an intricate part of the story, it wasn't the driving factor. Up until episode 5, we really haven't seen her be a true PI. I wanted to see some crime noir and mystery. People poo poo the idea, but I wouldn't mind a villain of the week type storyline.
I'm interested to see how it turns, but this is no way near as good as DD.
Oh and I've really liked Luke Cage. Hopefully they push him a bit more in the second half of the season. He's a decent character and at times the only one I like.