Yes I’ve used them. They have often seemed to help, and I’ve not had negative effects except when I’ve applied too much for too long, in which case it can burn. I’ve no doubt that they do have effects, since they are chemical and have strong smells and can cause pain (if it causes pain on your skin, it may not be appreciated by bacteria…). Strong-smelling ones under the nose can clear sinuses, and sinus pressure can cause headaches.
@canidmajor What bug repellent do you use?