Purchasing  new factory unlocked cell phones seems to be the way to go these days for various reasons. The most obvious impediment to using unlocked phones is that the option is only available to those who subscribe to cellular networks based on GSM or Global System for Mobile Communications technology. In the US that means carriers like AT&T & T Mobile.  Phones that connect to such networks carry subscriber and network information on Subscriber Identity Modules AKA SIM cards which can be easily inserted into the phone. If you own an unlocked phone and switch carriers  you simply take out the old SIM and put in a new one. I buy phones frequently , that means I can get a new phone whenever I want  without being locked into another upgrade contract.  PDA or Pocket PC phones that come factory unlocked have to be manually configured with acess ports for the internet and mail servers to function , your carrier can supply you with that information.  Now the best reason for me to use unlocked phones is to save money when traveling internationally. I do a lot of traveling abroad and U.S. cellular operators typically charge high roaming fees when their subscribers travel overseas and this can be hella expensive.  But with an unlocked phone when the plane lands I buy a SIM card at my destination  pop it in the phone and pay local rates in which ever country I happen to be in at the time and believe me that's far less expensive.  The downside to swapping SIMs when traveling is that your phone number will be different in the country you travel to so you will have to give friends and family your new number.  But for frequent travelers like myself this arrangement still has the cheap advantage.  Bare in mind that while unlocked phones are legal your carriers  operators and technical support are under no obligation to assist you in any way with configuration or any other technical issues because the phone didn't come from them but thanks to Google most of the information you will need is available through the web.  The last great advantage to buying unlocked cell phones is the wide choices you have in phone models.  There are a lot of awesome cell phones out there like the Nokia N80 , HTC Touch and the 02 XDA Stealth which I own and is quite the attention getter , everyone wants to look at it and play with it.   When you buy unlocked cell phones you have the option of owning the best phones the world has to offer and phones that most US carriers will probably never pick up and support and we can thank the FCC for that one. I can remember when cell phone technoligy was first introduced in Asia and Europe , for some strange reason the FCC would not approve them for use in the USA for almost 9 years which is why America will probably always be 10 years behind in cell phone technoligy , I have programs on my phone that I can't even use because American networking dosen't support those wireless applications yet. The biggest disadvantage to buying unlocked cell phones is the cost.   If you purchase a unlocked cell phone like my XDA stealth which has no US carrier backing you are going to pay full price , no discounts , no rebates no nothing , just a hefty price tag.  In closing I would like to offer one last bit of advise simply because I've seen a few of my friends burned on E bay.  If you are considering buying any cell phone with CDMA technoligy such as Sprint or Helio , be careful buying one from someone you don't know.  If the phones ESN number is not cleared for reasons like an outstanding balance on the previous owners account which is most common  , you will not be able to activate the phone until the balance is paid off. So basically now your stuck with a phone that you can't use unless your willing to pay someone elses bill.