I have mid-back-length hair that is very thick, and most swim caps just pop off my head. This one actually covers my entire head, forehead to nape (including ears) and has plenty of room for my hair.
It stayed on during my whole workout in the pool, and it's easy to put on and take off. Since there's no elastic band (like lycra caps have), it didn't leave a red line on my forehead. Basically, it's everything I'd want in a swim cap.
Some reviewers are interested in whether it keeps your hair dry, and the answer is that it mostly keeps water off your hair (I use a swim cap to avoid chlorine damage) but I wouldn't trust anything short of a magical force field to keep hair totally dry.