Page 530 in the Logic manual describes the tempophone effect, and you can find some history about the original tempohone on the net eg:
I don't think its intended to be as functional or invisible as normal flex. Its a special effect, I think its up to you to experiment with on whatever material you want.
I've used it before - I made three copies of a 4 bar loop, then stretched them all out to 2 minutes using three different flex algorithms. One of them was tempophone. The end result was great, the loops turned out totally different and sounded really sonicly interesting when played together. They were like three unique variations on the same theme.
I'd love to try it out on some vocals actually - tempohone creates a lot of artifacts so it could really make things sound interesting.
Try some experiments Victor and let us know what happens!