I acquired this travel mastermind version after having played the normal Mastermind game (and some of its variants like number/word mastermind) when younger. If you want to know the rules etc, then read this good review of that game:
http://www.boardgamegeek.com/thread/289843
The only difference in the travel version is that the code is only four pegs, not five, and there's only space for six guesses.

But as travel games go, it's not great for two specific reasons.

1) Fiddly bits. Mastermind involves lots of little tiny pegs that you have to stick in holes. Unless your manual dexterity is considerably better than mine, the chances of you playing this on a journey without dropping a couple of the pegs on the floor and never being able to find them again is about zero. You also need to hold it together with a rubber band, or the game slips out of the cardboard cover, and the bits go everywhere!

2) Small size affects gameplay. In an effort to make it travel sized, the code is only four pegs (which makes it easier and therefore less fun if you want a challenge). And there's only room for six guesses (which is bad if you're playing with someone new to the game or with poor logic skills, who needs more guesses to get it). It's also too few guesses if you want to play the variant suggested on the 'geek under the main game's heading:
http://www.boardgamegeek.com/thread/263243

So - it's hard to play, and has been dumbed-down in order to make it fit your pocket. Those are the problems.

On the plus side, it really is a pocket-sized game. It could fit in the pocket of your jeans. So perhaps it's better to describe it as Portable Mastermind, not Travel Mastermind.

It's not great playing when you're travelling, but it's easy to transport. For me, though, that's not enough when you add the reduced challenge of the gameplay. Playing this version makes me wish I had the proper version. Or wish I'd chosen a game more suited to travel in the first place.