I used to use them a lot (mainly because the models are amazing, seriously GW please give us a Possessed kit at that quality), but have steered clear of them for a while now. +1S just isn't super useful in my view because Possessed are already S5 and Venomcrawlers already shoot/hit at S8. Their respective leaps to S6 and S9 are rarely meaningful. A Master of Possession is going to be a far better accompanying character for a block of Possessed than a GP if you're looking to shove them all in a transport, where space is at a premium. They can also make you vulnerable to the Assassinate secondary if you take more than one, which is worth keeping in mind. This is the main reason why I don't think spamming them is a great idea, despite their CC prowess.
When I have used them, it has been with my Word Bearers. Having Revered Hosts on standby (+1D) means they can occasionally be a nasty surprise for opponents, but RH is better used elsewhere on units like a Lord Discordant with Exalted Possession or a big block of Possessed, which is why I dropped them.
They're not a terrible unit though, by CSM standards - GPs pack a great punch for their cost. But I think their usefulness is mainly tied to their aura, which isn't been particularly impactful (at least in my experience). Will be interested to hear if others have had success with them.
Edited by Marshal Loss, 10 May 2021 - 10:20 PM.