Necroids adds a new type of portraits that try to invoke death/undead feelings. They are really well done with lot of varied from vampires , zombies to dealth cultist.
THe new 3d models are also very cool to look at!
The major issue of this pack is the new playable mechanics. 1 new origin and 3 new civic traits. While this new content is good it seems to be lacking when compared to other portrait packs such as machines and lithoids.
It still has more content that Plantoids and humanoids.

I'd say if you want more portraits to have a more varied galaxy this pack is exactly what you are looking for.
If you want to experience a new way to play Stellaris it might be wise to wait for a sale

Mostly cosmetic, and very good art style.
Also adds some civics and a new origin.

First of all, the art for this one (specialy the ships) is amazing. The concept is cool as well, and it adds some unique playstyle which is always nice (more diversity is always good). Only sad thing is that they (imho) missed a chance of possible adding a new ascension type as well. But overall, a good DLC for a solid price.

The patch that came with the DLC was also a very much needed patch that improved the AI a lot compared to how it was before this patch.