I missed Justified in its original run, and always hoped to go back and try it because of the good things I've heard. It came back to Hulu recently, so I decided to give it a try. On the whole I really enjoyed it. It's the sort of show you can obsess over. I liked all the recurring character arcs - their story continuity was exceptional. This holds up really well in a streaming era.
I struggled with the first half of the first season. Raylan alone was not interesting enough to me. Bringing Boyd in to complete the other side of the picture was what made me keep going. I saw a piece of trivia on imdb.com that they originally intended to kill off Boyd in the first episode. I'm glad they changed the plan, because without the adversarial dynamic between the two of them I don't think this would have worked nearly as well.
I didn't always like Raylan. Timothy Olyphant's performance grew on me with time, but the character was arrogant and often aimless. In the end Boyd was the most compelling character to me. Walton Goggins was really, really good at his different moods and personas. You could see when he switched gears mid-scene from one tactic and mannerism to another as the character recalculated. They did a great job of setting him up against characters who were more despicable than him, and having him trying to achieve his own American dream. I could still often root for him, in spite of what he was. My interest in Ava varied, and I didn't care for the prison story line at all. I see others above commented that Season 5 isn't well liked, and I felt the same. All the marshals - Art, Gutterson, Rachel - were fun characters. I sometimes wish the show had spent more time on them. The recurring character casting was amazing, and I think it's a major strength of this show.
The Season 6 wrap-up was very good. I wasn't sure how they would find a way to bring things to a close, but they did as well as I think they could possibly have done. There is one aspect that I feel they glossed over - the lack of consequences for Raylan. I recognize that he's the hero, but he did a lot of shady things that never caught up with him.