NOTE: This is a reposting of a review I did over at myanimelist. If this seems a bit familiar, that's why. Additionally, I realized that my review isn't exactly friendly to newcomers to the series, so I've attempted to amend that.
I consider myself a pretty big fan of Katekyo Hitman Reborn. I love the manga; even though it took a sharp dive in quality partway through, I can still say it's worth reading. Another JUMP classic, Fist of the North Star, had the same issue and I think it's one of the greatest manga ever. So why am I giving the anime of Reborn such a low score? Simply put, it's a bad adaptation. While I haven't watched the entire series, I'm sure most folks will agree with me that 104 episodes of a show is more than enough for one to form an accurate judgement.
First off, I felt like the plot of Reborn was, for the most part, poorly adapted. The manga starts as a simple, goofy comedy series (a portion known as the Daily Life arc) that takes a hard turn towards action eight books in with the Kokuyo and Varia arcs. Anyhow, while I feel like the decision to move some of the Daily Life arc's more inconsequential portions around to act as fillers later on in between the Kokuyo and Varia arcs (as well as during the Varia arc) was an excellent idea, some of the adaptations of the plots in those chapters was, well, poor. Some jokes were stretched out and turned into something asinine (like the whole Stickypaper Theory thing), or removed entirely because the folks handling the adaptation felt they would be too risque for the audience they were targeting. One of my favorite chapters from the DL arc was one where Kyoko and Tsuna play Russian Roulette. Due to its content, it was left out entirely. (Truly unfortunate, since part of the DL arc's strengths lie in its occasionally morbid humor sensibilities.) That being said, several of these earlier episodes are still quite fun to watch. While some of the more humorous moments aren't present, these episodes have a "cozy" feeling to them.
When the anime got to the serious bits, it watered them down heavily to stretch them out. I know this is a standard practice for long-running mega-hit shounen manga adaptations, but there's no way those latter bits of the Varia arc should feel so slow, especially when the adaptation of the Kokuyo arc is, for the most part, fantastic. The pacing is magnificent; I never felt like it was dragging it's feet like the later arcs do.
Additionally, the staff eventually begins engaging in one of the most awful shounen manga adaptation tropes: the beginning-of-episode recap. I understand that these shows live and die on being able to maintain a fanbase, and part of that comes from being able to pull in new viewers. That makes these recaps in longer-running shows something of a necessary evil. However, these recaps can easily be much more concise; the only reason for their length is to help pad out each episode to help ensure the series doesn't catch up too far to the manga.
The worst practice used, however, is putting the tail-end of the episode AFTER the ED in some episodes, which I think is just plain dickish. The only reason for that is to ensure viewers watch the ED (which is advertising some song that the studio and producers earn money for advertising) and the commercials. While adaptations like this are, essentially, commercials themselves for merchandise related to the series brand (such as the manga), part of that merchandise are the DVD sales of the series itself, and that's doubly true for fujoshi-focused series like this. As such, placing the conclusion of the episode after the ED only serves to frustrate its core audience, as they may skip that last little sliver by accident.
The only thing I felt the anime improved upon is placing the chapter where a particularly plot-important character first appears much later than it occurs in the manga. I actually had no idea he had appeared about 124 chapters earlier when Amano reintroduced him.
Moving on to toning down the darker bits, that move was fairly irritating. The fights lacked a lot of the intensity they had in the manga due to the decision to stop having characters bleed after the Kokuyo arc. In addition, Artland's decision to make all the weapons look unrealistic was fairly stupid, but I can tolerate it for the most part. I can buy having Leone turning into a pistol or a rifle for Reborn to shoot, but the whole "Dying Will energy leaving the bullets and flying into Tsuna" thing was idiotic. Why didn't they just remove bullets leaving the guns and replace it with the energy beams? And while I can tolerate replacing missing teeth with pocketwatches (it makes sense in context; I'd rather not spoil anything), it removes a lot of tension from those scenes.
Most importantly, they removed Yamamoto's introduction. While I won't spoil anything, it's probably the most moving character introduction in the DL arc, and Artland cut it out entirely because they felt it was too dark. Now, Yamamoto just looks like a moron that suddenly begins tagging along with Tsuna and Gokudera because he's intrigued by what he believes to be their mafia role-play game.
Moving on to other character alterations, Tsuna's quite a bit less of a jerk early on than he was in the anime, and is mother is much less mean to Tsuna. Considering Tsuna's behavior ealry on, her attitude makes since, and it later softens as Tsuna (despite what Akira Amano may say in the manga's God-forsaken non-ending) grows as a person. In the anime, this is removed entirely, as well.
The art actually looks pretty damn good; and, in the DL arc, it's a great improvement. The animation, however, is not. It's very jerky and still. Amano's artwork in the manga is far, far more animated than the anime itself is thanks to some of her wonderful panel layouts. Many of the actions scenes look absolutely sluggish in comparison to the manga. Artland could've attempted to fake movement by making the artwork more dynamic (which is arguably the backbone of Japanese animation) but they couldn't even be bothered to do that. Instead, every fight looks terribly bland and overly clean, with the characters being drawn exactly on-model, virtually without injury, and rarely moving.
I'm a bit baffled on what to say for the soundtrack for the show. I should note that I'm not referring to the OPs or EDs, as I generally skip those; while okay, they weren't worth hearing repeatedly in a marathon of the show. There were about two fantastic tracks in the shows catalog: Standing Friends and Tsuna Awakens. However, the anime greatly overuses them to the point where they lose almost all meaning. The other tracks are fairly mediocre, and, as such, don't deserve much criticism at all, positive or negative.
In the end, I can't recommend this anime. I hate that I wasted so much of my time waiting for the anime to start adapting the manga right, and it never did. To this day, there are fans in Japan (http://otakuusamagazine.com/LatestNews/News1/Japanese-Fans-Want-These-Anime-to-Get-Sequels-8399.aspx) hoping to eventually see the remaining two arcs (one a step-down in quality from the one before; the other a nose-dive) adapted. I can only assume the reason for wanting more of Artland's adaptation is simply so that its obsessive fanbase can hear the characters speak even more, which, honestly speaking, is probably why any of them followed the anime in the first place. It does have fairly solid voice casting, and that is probably the cornerstone of the anime's success. Several image albums featuring the cast members singing several songs related to the series (tangentially or otherwise) in the voices of the characters were released, with one of them even becoming an ending song on the show. Most notably, there were about five different "Rebocons"; concerts where the voice cast would act out skits on-stage like the characters from the show that they portray and sing songs. I can't say I've heard of other VA-centric series like DiaLovers, for example, courting followings like that.
Anyway, read the manga instead. It's far superior in nearly every way. The only conceivable way the anime is better is if one is a massive seiyuu fanatic.