Well, I just downloaded my copy this morning, and I'm overall quite happy with it - mainly because it makes some units I particularly love a bit more viable. Here are some highlights:
Lictor Forest Broods: 5 lictors instead of 3 - get Shrouded instead of Stealth in forests and can Infiltrate w/in 6" of an enemy model in forests.
Manufactorum Genestealer Formation: 5 minimum-sized broods of Genestealers (5 apiece), gain Hit & Run and may be deployed w/in 6" of an enemy model if infiltrating into a building or ruins.
Deathleaper's Assassin Brood: Deathleaper and 5 individual Lictor Broods - gain Preferred Enemy for characters, and any enemy unit within 12" of at least 1 model from the formation suffers -1 to their Leadership (so no, not "stackable")
Broodlord Hunting Pack: 3 broods and a Broodlord - only 1 Broodlord per formation. Can arrive from Reserve in an unoccupied Building or Ruins (at least 6" away from enemy if in Ruins). Select an enemy unit when the Broodlord is deployed - all models from the formation get Preferred Enemy against the nominated enemy unit.
Gargoyle Bio-Bombs: Fomation of 3 spore mine clusters & 3 Gargoyle broods - spore mines get better movement if they start w/in 6" of a Gargoyle unit from this formation (move 6" in the Movement phase and does not halve distance for running or charging).
So what does this mean? Personally, I love Genestealers - and I think the two formations above make them a LOT more viable to use on the table-top. Most tables have buildings or ruins as scenery, so they'll have plenty of choices of where to "pop up."
I can't see me using the Assassin Brood, but the Forest Broods might be tasty. I dunno...
Before, I thought Spore Mines were pretty much unusable - now, they might be worth another look.
So do these new formations change your opinion on 'Nid viability in 6th? I think if nothing else, we might be seeing a lot more Genestealers doing the "Ymgarl" trick and popping out of scenery - that's the way I'm intending to use 'em, anyway!