I just tried FaceNiff on another PublicWifi, where you actually can connect, but have to pay to get outside connections.
Now the interesting part: While FaceNiff was running, i could surf on that hotspot... without logging in. I could even access the internet from my notebook, when that was also logged into the wifi.

Anoter time, when i started FaceNiff, it completely jammed the network. Suddently the internet wasnt working anymore, for everyone on the network (pretty crazy faces around me)

Whys that happening?

It bends the hyperspace!
In fact it uses arp spoofing and it could made router do crazy things.

Im the same i have a few services around me .. When connect to it i get the default login page and you can signup to use there wifi for free. I wonder if its neccessary to signup at all or if you can sniff users anyway