I have on few occasions seen PDF files where the text looked horrible in pdfjs. All in all, it's very useful, though, to quickly look at a PDF before saving it.

For regular use, I have come to really like SumatraPDF on Windows, it is relatively lightweight can be used without an explicit installation (hence no admin privileges are required to get it to work), and most importantly, it saves the position on opened PDF files, so if I open a file again later, I am back right where I stopped reading.