You can't use them if you have a saddle bag, won't fit. I bought one a while back but haven't actually used it yet as I avoid riding in the rain where possible and also the aforementioned saddle bag issue. Plus the few times I have ridden in the rain, I didn't plan to do it, the forecast said fine so I ride to work and then in the afternoon when it's time to go home it starts raining and therefore I didn't have my ass saver with me.