I haven't used one, but here's somewhat how they work.
It works like this: A small audio speaker or high end driver is enclosed, air tight, into a metal box with a hole in the top where a tube comes out. The guitar is plugged into an amp, but the speaker output is routed through the TalkBox. Then the signal is routed back from the box into the amp again. So, the sound from the guitar is sent through the box, up the tube to a waiting mouth, where the sounds are manipulated by Peter, before being picked up by the microphone! So by this method you can mouth the words while the sound of the guitar replaces the human voice box....hence the name TalkBox! Of course, Peter doesn't use the effect on every song........but you can hear it lately on the album 'Frampton Comes Alive II' on the song 'Can't Take That Away'.