As an expression, it is fine and well recognised. But the individual words are otherwise uncommon in everyday language. You would be more likely to use something like "warned" or just simply "told" on the one hand, and "prepared" on the other.

Very common saying in American English. => Forewarned is forearmed. New Hire: What's the boss like? Old guy: He's okay as long as you look busy, but if he catches you slacking off, you're in for a chewing out. New Hire: Forewarned is forearmed. Thanks! Also, A word to the wise is sufficient. That's good to know! (thanks for telling me in advance) Better safe than sorry. Better to have it and not need it than to need it and not have it.

Mishelle,thanks for your help!

For Americans, I dont believe that we use that expression in casual conversation. I can't think of a similar expression that we use either.