Any time you are on public Wi-Fig, you are at risk for hackers seeing your personal information... Credit card numbers, user names, passwords, etc. If you have a data plan for your device, you can turn off the Wi-Fi and you will be communicating directly through your data provider. This is much less risky.
I normally do not worry about too much other than banking. I should probably do the same for this forum. If someone hacked my F4F account, you guys might like them more than the real me.