I've used both, although I regularly use only arm warmers. I find that they're helpful in spring and fall when there are big temperature variations between the beginning, middle and/or end of a ride. I wear my arm warmers the most when cool mornings (mid 50s to mid 60s perhaps) give way to warm days. I have worn them on colder days, however, in combination with a with a jacket or long-sleeve jersey.
In reading recent posts, it seems that some people prefer knee or leg warmers to knickers in terms of versatility. I've just never found a pair I like as well as knickers. I generally wear my knickers in the low 50s and up. Colder than that, I wear tights.
There's really no one way or right way to use them. They're just a versatile addition to your wardrobe in that they stash pretty easily and let you transition during a ride. They come in various weights and fabrics, too. My arm warmers (by Sugoi) aren't too thick or too thin.
Live with intention. Walk to the edge. Listen hard. Practice wellness. Play with abandon. Laugh. Choose with no regret. Continue to learn. Appreciate your friends. Do what you love. Live as if this is all there is.
--Mary Anne Radmacher