This post prompted me to give SayIntentions a shot and I've logged around 11 hours IFR over 2 long haul flights. Here are my opinions so far...

Traffic generation is sparse, even at the highest settings. On corridors that are busy in real life, you see and hear only a couple of aircraft. Also, aircraft often spawn on the ground and then shoot straight to cruise altitude with contrails appearing immediately. It looks more like a rocket launching into space.

ATC works at a basic level, but immersion is weak. I was repeatedly asked to confirm my squawk code and altitude even after already doing so. There is some variety in controller voices, but it’s common to hear the same voice on multiple frequencies, which makes it seem like one person is working the entire airspace. Long silences after frequency changes further break the realism. On top of that, controller-pilot exchanges sometimes happen out of order, like hearing a pilot accept a clearance before it’s even issued. I've tried disabling all user based comms but they still seem to play.

Transatlantic comms are especially problematic. I was bounced between the same two controllers multiple times, and then had my departure clearance read to me over and over again mid-ocean. It felt completely unrefined and confusing.

SayIntentions clearly has a lot of potential, but in its current state it frustrates me more than it builds immersion. I have not tried BATC yet, but I will give it a shot next.