The show is great. And rewatching the past seasons of Samurai Jack is not required although it would enrich the viewing experience.
The current season blends Mad Max style of story-telling with less bombast and adrenaline. The animation is a marked improvement to the previous seasons while not so jarring (a la Xiaolin Chronicles). But personally, Jack's new design (both unshaven and clean) felt a bit weird to me.
For older fans, there are easter eggs and references galore but it does not hinder the story for any new viewers. The change in Jack is like the change in Max between Mad Max Thunderdome and Mad Max Fury Road. We know he's been through a lot and is suffering from PTSD which makes want to root for him more to beat it.
Lastly, the atmosphere that engrosses you into his story remain intact. The beautiful backgrounds, the raw emotions and new mature themes all make it an awesome show to watch.