I have a pair of knee warmers but I haven't used them in years. They were perfect for commuting though. I'd wear them to cover my knees for the ride in when it was cold, but then could leave them off for the ride home when it had warmed up.
I don't wear arm warmers because I've big upper arms and I generally find them uncomfortable. I do have multiple boleros though and I like using them for climbing rides where I'm warm on the way up but will be chilled for the descent (put them on at the top). They'd also be good for long rides that start early where it's cold but continue until it gets warmer in the afternoon (same for knee warmers) so that you don't need to carry a lot of extra clothes around with you.
Edited to add: My knee warmers are from Canari and they are very nice. My boleros are from Terry and Sheila Moon and I like both styles equally as well though the Terry one is thicker/warmer for winter use.