I have to say Reborn is one of the mangas I've liked the most out of the many I've read. What I liked the most, and the reason why I even started reading it is because of the genre shift that made Reborn so famous, and it's something really amazing. It really shows you how the story grows and changes from the original lighthearted and quirky gag manga to a darker and more action based version with great characters and developtment.
Tsuna is one of my favorite characters in all of manga because of his growth from a shy, insecure and cowardly boy into a....braver version of himself, he's still pretty shy at the end but he's more open about speaking his opinion and isn't as much of a coward as he was before.
The fights were great, though they started to pile up. This relates to the plot. For example, in the Kokuyo arc after the shift the fights are in a realistic level with some few points that sound too weird to be true as more supernatural stuff are added into the mix.
What I liked most about that arc is how everything in the gag portion seemed to come together to the final fight against Mukuro which showed just how much Tsuna had really grown.
As the story went on and the arcs came out Reborn began to escalate and create its own little plot, and this is both good and bad. While the fights start off with few things that are out of this world it eventually gets even more complex as the story unfolds, it's not like it was bad but there came a point where you just didn't know where it was going to go.
This was more evident in the last few arcs, with each of the arcs starting out as the most amazing thing that would happen and ending up in a sort of rushed conclusion or as a dragged out fight, of course that's probably due to the fact that reading it week to week caused the fights to grow old even if they were pretty good.
What I think the problem was is that Reborn tried to bite a lot more than it could chew and when it was finally time to close all the ties it didn't know how to. I also didn't like how it ended very much because it felt like the author just wanted to go back to gag because she didn't know how to end th emanga properly, so in the end we came full circle, except that Tsuna seemed to have forgotten everything he learned.