It is a good idea to purchase a sizing kit before ordering the Oura ring, so you can properly test the size first (wear it for at least 24 hours). I initially thought I needed a specific size, but apparently my fingers swell up a lot at night. Without the sizing kit, that size would have been a major mispurchase.
The ring is very pleasant to use. The app provides very clear insight into my day. Because I have energy issues due to Long Covid, this overview helps me enormously to determine when I need to rest. The ring tracks your sleep, restedness, heart rate, steps, calorie burn, activities, breathing, and much more! Additionally, it can also be used underwater, I believe up to 100 meters.