I wonder if we will have full control of transparency z sorting groups, like you have with actor components…
I note that the niagara branch changes, of which there are many, got merged into master on github very recently, so looks like a great time for curious people who like the bleeding edge to take another look. I might try later today, though since I never tried it in the past I am starting from pretty much zero knowledge.
And dev-niagara branch is publicly available now, for people who are even more… curious/impatient 
/EpicGames/UnrealEngine/commits/dev-niagara
I dont have much time at the moment but I did get as far as compiling that branch and determining that there is enough content in the Niagara Extras plugin to see something working without having to learn much of anything about the intricacies of niagara (I found a swirly thing and a skinned mesh recreation before I ran out of time).
Today i was streaming showcasing and playing out with niagara branch 
it shows:
-Explains basics
-Shows massive interactive clouds of GPU particules
-Working collision (thanks for Niagara developers in chat explaining how to make it to work)
-Mesh recreation
-Curve sampling
-How to make modules kinda ;p
It little bit unorganized as it was a stream