Yeah, it's fine. The legality is a little fine, but it's largely positively viewed.
The sites themselves encourage people to buy the manga once it's released regionally, and the (Japanese) producers of the original manga like that these sites "whet the market" in western countries. Hell, I've even translated manga, myself, one of which you can find on onemanga.com, at that. Personally, I'm not into manga, but translating was good practice during my Japanese studies.
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Oh, well, fancy that! Isn't that neat, eh?