Soothing, really. I've never in my life seen an anime — or any video media, but definitely an anime — come across so calming. I could easily fall asleep to this and it's not what I expect at all.
Being not a huge anime nerd, I don't know how well-known it actually is or not, but it's always nice to see it being mentioned
Pretty well-known. It's obviously not Pokémon or whatever, but if you go on anidb.net and sort all anime by rating you'll get Mushishi third (and sixth and tenth).