Certainly the closest we have received to a Half-Life 3, this game is actually a full game(10+ hours, I think?).
The catch is this. The game is only available in VR, something I can not see myself getting any time soon. It is apparently the first killer app for VR and the reviews online are through the roof(IGN gave it a 10/10).
Has anyone had a chance to play it? The Youtube videos of people playing are a lot of fun, but I’d love a chance one day to play it for myself.
Yes. I’m not very far into it yet, but it’s amazing! I agree that it has the potential to be the killer app that finally gets VR off the ground.
Here is a GIF of a math teacher using that game to record a geometry lecture: HLA Maths GIF | Gfycat
YouTube video: Let's Play: Angle Vocabulary Review (in Half Life: Alyx) - Supplementary, Vertical and Complementary - YouTube
My wife is a university math professor, I showed her that YouTube video and she forwarded it to some of her colleagues. 
That video was recorded right in the initial starting location, I also discovered the markers and spent a lot of time drawing on the windows (you can write on the video monitors, too).
The attention to detail in this game is incredible. Little things, like you can pick up a full bottle of beer and if you shake it near your ear you can hear liquid inside. I spend as much time walking around and looking at things as I do actually advancing the game. For me, it has bumped Elite: Dangerous down a notch for the most immersive VR game I’ve ever experienced.