With sunscreen, there are two key usability factors. Firstly, how easy is it to apply? Second, how comfortable is it to wear?
I'll be honest; there are sunscreens that are so annoying to apply-maybe they don't rub in easily, or they leave your hands covered in an oily film or sticky residue-that I think twice about using them. And, if the sunscreen runs into and burns your eyes when you sweat, well that can just about ruin your day.