My phone screen is >600ppi. My e-book reader (e-ink) is 300ppi. For me 300ppi seems to be the point at which things start to look "pixel-less". A 300ppi e-ink display looks remarkably close to paper print quality. I think popularity of sans serif fonts online is mostly due to serif fonts looking shit on low PPI displays. It would be really nice to have a 300ppi monitor just so I could switch to more pleasing fonts.