It's really worth it. The first has an incredibly detailed area with new enemies and gear, a great final boss, and a bunch of optional content and a boss to unlock the PvP arena they added.Damn, sounds like I'll have to replay DS3 sooner rather than later.
The second has two full areas with new and difficult enemies, new gear, three of some of the most challenging and best bosses in the series (including a perfect final fight), and another boss related to the new covenant. It's a spectacular finale.