Both plugs are soft silicon and are comfortable inserts but the proportions and plug design are not great. The ratios between bulb size to neck diameter and neck length are all wrong for this to be a comfortable plug to wear around for long periods of time.
The bulb's circumference compared to the neck on the Small is only an inch difference. The Medium has a better ratio and fits better. These are good for stretching, but not great for retaining while active.
Both the plugs have short necks (Small 0.5 in. Medium ~0.75 inch long) so they can be tight depending on your anatomy and can cause you to feel like they are pulling out of you on their own.
The round base contributes to this as well as the cheeks are being dug into and can pull on the plug too as you are moving around.
Small - 2 stars this seems poorly shaped for retention.
Med - 3 stars better shape for retention but the base prevents it from being better.
Overall - good for stretching before playing with other toys, not good for any daily activities or moving around.