Bought the small size for my Watts EPS helmet. The sound is fine, and the volume control on the cord is convenient and easy to use if it doesn't slip down into your jacket.
The main problem with the liner is the placement of the attachments for the inside of the helmet. It snaps in at the back and the sides, but there's no attachment at the front. I fixed this by using a small piece of velcro strip from Target to make my own attachment point. Secondly, the actual speaker in the liner didn't line up exactly with my ear on the left side, and after a full day of riding it was pressing pretty hard on the cartilage, resulting in some soreness.
All in all though, I'd say it's worth it. Saves your brain while playing cool jams.