I LOVE my arm warmers, but I also use them for running, so they become a little more indispensable than if I were just wearing them for riding. For riding, they are most useful during fall and spring. Once the weather gets too cool for riding, I switch to heavier weight long sleeve jerseys and/or baselayers and/or jackets.

I have yet to get into knee/leg warmers - though I considered it when we were on our fall cycling tour. I brought them and wanted to try them out, but then the weather got way too warm.