You don't need this service if you get a GSM mobile service provider, such as Voicestream or AT&T in certain areas, and what is known as a tri-band phone (I think Motorola may still be the only manufacturer though to offer this).

Then when you are overseas, your phone will work and roam with very little problem. You keep the same number, and the best part is that you don't have to pay extra or get new numbers or anything. Someone dialing your US number will get you wherever you are. Since most of the world is on GSM, it is quite convenient. Not so convenient if you travel outside major areas in the US, but the great otherwise the world over. And as mentioned, you only pay for actual calls you make while overseas...no daily roaming fees or anything like that.