Is it the volume version I am downloading or does the manga art quality drops radically, being no more than rough sketches?

well sometimes the author doesn't put a lot of effort into the art, like at the end of the creed island saga and it is just rough sketches, but most of the time the art quality is very good.

During the middle of the Greed Island arc the mangaka was sick or something, so the art quality went down (though I think you can find the cleaned-up versions that were scanlated). It's back to normal now, though, so don't let it put you off. HxH is one of my favorite series of all time!