I'm assuming that you are talking about TFN as in the contract not the copy.
And yes I use it, in a small market like i'm in, we are very informal... So once I know I have a good relationship with a client, and they have seen results, and we have followed through on all our promises, I ask if they might be interested in going TFN.
Then I explain that there is no obligation, and they can cancel at anytime, but this is to make things easier on them, this way they won't have to keep debating contracts and prices with me, we can agree on something and run with it.
(we usually get new copy every month.)