It's a personal preference. OP is not some kind of omnipotent series that encompasses all genres perfectly, such a manga doesn't exist. People who prefer horror, slice of life or psychological manga are probably less prone in liking OP. It's something personal and subjective.
That OP is more than a decent manga however, is something that can be more or less objectively supported. By default I assume that people who describe something like a manga as "trash", are people who are borderline illiterate, immature and are below the average human intellectual level. They reason if they use a word like "trash", which either has a derogatory meaning or refers to something disgusting, they can shock and insult. The reality though is that it's a hollow remark with no substance and is just another way of saying that they don't have the skill to give elaborate criticism, so they try to shock people to mask that.
IMO no manga deserves to be slandered like that. There should be something seriously wrong to deserve such a description and the reality that someone decided it was worthwhile to serialize it, and keep it serialized, means that that really is not the case.
And in regards to the anime, well it has some issues, but those are not inherent to OP as the exact same issues can be found in Bleach, Naruto and other animations. When you make an anime adaptation of a lengthy, running series, it's inevitable that the animation quality will not be that high and the problem of catching up to the series is permanently present.
