Text to speech is far more wonderful than most people give it credit for, and the Kindle text to speech is among the best you can get right now. I seem to be in the minority, in that I greatly prefer the woman's voice (her name is Samantha, btw).
I am looking forward to more voices being made available. If you want a taste of what's in store, try the samples at
www.nextup.com. Text to speech is not just good for when you're traveling, it's good for any time. It is superb for helping pull yourself past text that you have a hard time reading because, say, it's boring. It is excellent for a lot of special purposes, and the more choice of voices, the better it will be.
Many books that I have purchased allow text to speech, and all of the many I've gotten for free do.