I've never needed one - however kinky the play has been (from "not-at-all" to "slightly") it's never involved anything that would confuse the communication with my partner. I try always to be sensitive to her needs and mood (as far as I can tell) and a "No", "Wait" or "Stop" always means Stop-right-the-hell-now-and-find-out-what's-the-matter.
Honestly, as fun and exotic as it might be to need a safe-word, it would be perfectly possible to enjoy a seriously kinky relationship without using one, so long as each of you knows you can trust your partner to listen carefully when you tell them things.
However, the minute your play starts to involve roleplay, D/s mindsets or any kind of scenario where you might say something for effect rather than information, I think a clearly-designated safeword or two becomes essential.
Maybe I need to think up some now, just in case...