It is common enough to use in conversation where appropriate. I think the adjective form, "whimsical" is more common than the noun, "whimsy." To me, "whimsical" means imaginative, like something from a dream or a fantasy. It does connote lightness, airiness; not tied down by logic or reason, but in a positive way. To use it in a sentence: This children's book about a boy's adventures with a unicorn is quite whimsical.