Neural is situational and a lot of people decide that it's not worth getting the upgrade because of that. However... Neural parasite on immortals, archons, and thors yeilds great results. The problem is they rely on unit formations and how the enemy is controlling their army. Still, if your patient and let them make mistakes, we have seen it yield great results even at the pro level.
It's a relatively cheap upgrade and has the potential to swing battles. Just don't suicide them trying to neural the units on the back line. Dance around until you get the engagement you want.
This is absolutely true. Not many realize how devastating it is to have an Immortal or Archon turn on your army. It's more devastating IMO than having a Colossus turn on your army simply because it's not as easy to find where that Infestor is at, if you have to kill your own unit than you need Zealots (which are in the frontlines) to kill the Immortal and Stalkers (which need to focus on Brood Lords, Roaches, or Corruptors) to deal with the Archon.
In WoL, it also makes PvZ Lategame with Mothership incredibly terrifying. If you're Protoss, it's probably the biggest reason why you lose to lategame Zerg if you've followed the necessary steps to stay in the game healthily to that point.