You can watch it stand alone NP, although a bit of the opening sequence of the first episode may confuse, plus you'll miss out on the ...not back, but forward story of a couple of the main characters.
For those watching, I agree with ROC and Ripsnort. ROC - have you been wondering what ever must have happened with Kim by the time Breaking Bad happens, as she isn't featured at all, yet is a major part of the protagonists life in Saul? Foreboding, I have a bad feeling for some reason, which is keeping me glued to this, as you said, great sleeper.