I've got the Outdoor Research Helium bivvy. Useful features include the short arcing pole that creates some space around the head, and an inner fly sheet at the top end. This latter feature means that the outer zip can be left open when its not raining, allowing more airflow. Even when it's raining I still leave a little gap, for peace of mind that I'll still have oxygen through the night!
I've used it half a dozen times. It's been perfectly fine, although I've never used any other bivvy bag so can't compare. The first time I used it I was in the open, and a thunderstorm came through. It got a little damp inside, but everything dried out quickly before I setup camp the following night.
I've noticed that the 2022 model is slightly lighter than the one I have, but in other respects it is the same.