EDIT: I made the following observations in a testgame vs. Orks a few days ago.
ok some feedback from me. I was used to playing the "old" list, with d3 swarming.
I have to say I don't like the new system at all. While I understand the idea to link spawning to swarm size to make larger swarms more interesting, I think that overall there isn't just enough swarming going on. A lone Tyranid Warrior who is the last survivor of a small swarm bringing back 6 bases might seem like too much, but consider that you have to move him out of enemy range (30cm). But the one base that is swarmed now is just rediculous, I am not getting a bug-swarm-overrun feeling with that

I say this also thinking about Necrons, who can bring bases back bases on their score on the rally roll. Ideally they can revive up to 6 bases, and we're talking about 4/4/4 Armour/CC/FF units here not Gaunts...
So my suggestion would be to do somenthing similar and tie it to the rally roll, whith maybe a fixed bonus per synapse unit.
Another thing is the WE limitation. While I kind of always played it like this, I only considered the Dominatrix and the Hierophant/-dule for it. Now I can't even field tunnelers and some Harridan because I go over the limit. I don't like this, in other armies you also have lot's of DC3 WE (think Baneblade) that are not tied into the 1/3 limit. This needs to be tweaked IMO.
FYI my army list usually looks something like this:
NEXUS SWARM [350]
Dominatrix
TYRANID SWARM (M) [575]
6 Termagants, 2 Hive Tyrant, 6 Hormagants, 4 Carnifex
BIOVORE SWARM (M) [275]
10 Biovore
LICTOR SWARM [250]
5 Lictor
LICTOR SWARM [250]
5 Lictor
HIEROPHANT BIO-TITAN (745.M41) [525]
TYRANID SWARM (S) [200]
Two Tyranid Warriors, 3 Termagants, 3 Hormagants
TYRANID SWARM (S) [200]
Two Tyranid Warriors, 3 Termagants, 3 Hormagants
to get to 3000 I then add Harridan, Trygon or another swarm with Hive Tyrand & Carnifex, and add a Zoanthrope here and there (I don't have any of the other models).
cheers,