I FINALLY finished watching Monster two days ago! It took me two years, but I'm glad I finally got around to it. I really enjoyed it. I've seen lots of people say that the ending was anticlimactic and sucked, but I personally enjoyed it for the most part. All of the characters in the show are fantastic, too. Even side characters such as Grimmer and Martin were very fleshed out and I'd say their stories were just as engaging as the stories of characters like Nina and Tenma.
I also think that Tenma is somehow a underrated character from what I've seen? I think he's overlooked because of Johan's popularity, as well as the fact that he's a main character. I thought he was a very well-written character that I thoroughly enjoyed. He was definitely my favourite (maybe I'm just biased for that reason, haha).
I'm trying to avoid saying any spoilers, so I'll be vague, but I think it was interesting when Tenma would disappear for multiple episodes. As much as I wanted to see him, I think these disappearances made Tenma more unpredictable when he does appear, like we can't tell what he's thinking so much. Same thing with Johan. I think this was a very cool choice that made their interactions all the more interesting.
So overall, I think that Monster does have some pacing issues and can be a bit boring at times, but it's worth it in the end. It's a very compelling story with fantastic characters that I would definitely watch again :]
I also caught up on the last few episodes of The Summer Hikaru Died today. I'm really enjoying it!