John Brandon, writing for Macworld:

Still, even if a hacker could snag your transaction data as it passes from your iPhone to the terminal, they’d get a single-use token with nothing to identify you by name. Connecting that to the credit cards stored securely by Apple might not be impossible, but the experts we spoke to agree that it’s a lot harder than just stealing some credit card numbers.

If you’re concerned about security it looks like Apple Pay is exactly the service you should be using.