Been able to activate it on Beta releases for several weeks now, but don't know anyone to use it with.
For those who are asking, APC allows you to send\receive cash between iOS 11.2 users via Messages app, regardless of what bank the other person uses. If you have a credit balance on your APC card, you can pay from that, else from any card in your Apple Pay wallet. If you have a debit card in your wallet, or if you use APC, there is no charge. If you send cash using a credit card, there is a fee.
All $ received goes on your APC, and you can transfer it to your bank via the debit card in your wallet.
It would be nice if the Mac Messages app allowed APC as well, perhaps like Apple Pay with using TouchID (or FaceID) on iPhone to authenticate the transaction... maybe on the 10.14 feature list??? Along with App Store for Mac Messages... one can hope!