(might contain minor spoilers, just to warn you)
Beck is a brilliant manga.
The first 2500 pages are great, you constantly get to see something new, showing the character/band development.
The main selling points of this manga are the character development and the emotions it manages to create in you even if you can't hear the songs.
It has it's weak points, too, for example, in the beginning it has a really weird art style, but that changes over time, I think it was gone about 2000 pages in.
It kinda loses it's pacing in the middle, the exciting points are more spread out than in the beginning and in the end.
The last 1400 pages are truly amazing, everything that was built up to that point truly comes to shine, allowing for a magnificant finishing marathon with a very great ending, wrapping up every loose thread remaining.
To finish it up, my 3 favorite scenes from the manga:
1. The whole avalon festival, but out of it, especially the moment when MCS was announced to play on the main stage in front of 100k+ people, nothing short of epic.
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Maho reuniting with Koyuki at the train station, very powerful scene.
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Koyuki getting pulled on stage by Dying Breed at their secret show.