They stay in well by themselves while lying down walking around and at gym but the only time I have to worry is when I put my hair behind my ear they get caught and are pulled out. Got to be more aware of that!
They fit just like the Airbuds. I don't understand all the drama. If Airbuds fit then air pods will too. For running or fitness maybe supplement with the speigan tool of the airpods covers. However a full time strap defeats the point of these!
Once when taking off my messenger bag the AirPod tail got caught by the strap and yanked out. I'm careful not to repeat that now. Otherwise they're quite snug.
Mine got stolen, needs anti-theft feature!
They stay in when stationary and loosen up when walking. I need to adjust them a lot.