I watched both the series and the OVA for The Violinist of Hamelin. I have also read about half of the manga. The two anime could not be any more different.

let's compare the OVA...

Both of them basically do their own thing rather than following the story. They kind of have to, since the manga is about 35 volumes long. The OVA is just one episode, but it basically follows the spirit of the manga fairly closely, even if it’s a made up story. It would basically give you a good feeling for what the manga is like, and then you’d go and pick up the manga to actually follow an extended storyline.

...to the series

The series was…yeah. It was almost certainly the worst anime adaptation that I’ve ever seen. Let’s go down the list:

For starters, the manga is a comedy. To be overly reductionist, it’s Nodame Cantabile meets Yakitate! Japan. “Oh noes, there’s no way that the good guys can win now!” “LISTEN CLOSELY!” “Of course! Sibelius’s Finlandia! There’s no way the good guys can lose now!” If they ever get serious during the fight scenes or whatever, without fail there will be something silly to puncture the tension, like a joke about a Gundam taking a shit. The anime, however is completely serious business. I can’t even think of any attempts by the show to be funny.

The cast really seemed like they were mailing it in compared to the OVA cast. This is heavily due to the serious business tone, which basically required everyone to say their lines in a monotone, serious business way to emphasize that fantasy battles are serious business. As opposed, of course, to the manga where fantasy battles mean that you put on silly disguises and launch people out of catapults. As for the serious business Megumi Ogata, for example sounded just like Shinji rather than like Sailor Uranus. Unfortunately, Sizer is a valkyrie, rather than a lily-livered boy with daddy issues. Meanwhile, the OVA cast actually you know, has fun with what is supposed to be a silly title.

The series takes 26 episodes to basically cover a single battle. It’s basically able to do this by stretching everything out while simultaneously having nothing happen. It’s really hard to describe how they do this because it’s so hard to remember anything specific when really nothing would happen in episodes at a time. I think I counted, for example six episodes where Hamelin wasn’t even present, and the series is named after him.

Lastly, the animation. OMG, the animation. The manga was more animated than the anime. The show would literally go for minutes at a time with no animation or even use tricks to make still frames look like they were moving. For example, they wouldn’t even draw speed lines going back and forth for a few frames and then looping them for 5 seconds while a character leaps up into the air. It would just show the still frame for the five seconds. There would be conversations between two characters, and they wouldn’t even cut back and forth between their faces. It would just leave up the same still the entire time. They wouldn’t even zoom or move the camera over a large image. It would just sit there.

Would I watch it even if I weren’t forced? Watch the OVA, then read the manga