I use them for my feet when riding and feet and hands when x country skiing at low temperatures. I have tried to use the hand warmers when riding, but I can't find any combo of liners/gloves that I have that are big enough. I can barely shift the gears on my road bike with my Amfib gloves that are really quite warm down to 30, so I end up wearing liners with lighter gloves, which make things pretty tight. It's OK with my other bike with flat bars.
I have liners for skiing that have a holder place for the packet, but I just can't get them to fit under my cycling gloves.