Positives:There are many positive things I can say about the HP iPaq 510. I brought this phone on my recent trip to Vietnam, where I would buy a prepaid SIM card, loading 30 minutes to it, and the phone just worked as expected. There are many unlocked GSM phones out there, but I'm still glad I bought this HP phone because of these three reasons:First, the HP iPaq 510 is light, compact, and it looks pretty cool. To other people, especially those overseas, the phone could appear more expensive than what you've actually paid for it.Second, it has Windows Mobile, which comes with many extra and useful applications, such as Task List, Calendar, Internet Explorer, and games. The phone also supports Bluetooth and wireless Interent. Using the phone, I can surf the Web at hotels, shops, and airports, wherever there's an open Wi-Fi connection. It's just like I'm carrying a small and convenient pocket PC with me that can make phone calls all around the world, wherever I go.Finally, this phone supports Skype, so once you download the Skype client to the phone, you can make phone calls over Skype wherever there's a Wi-Fi connection.When I'm back to the U.S., since I've already been under a contract with a CDMA cell phone company, I can't use this phone as my main phone. To make this phone be continually useful, I bought and installed a prepaid SIM card from T-Mobile, and turned it to my car phone. This phone now handles my occasional phone calls in my car, while waiting for my next overseas trip.In general, this iPaq phone is useful for me everywhere, anytime. I would recommend it.Negatives:Occasionally, the phone freezes up at boot up and shutdown time, forcing me to take out the battery and put it back to force a reset. Windows Mobile, the OS that operates this phone, apparently still retains the Windows' trademark of unreliability from time to time. I hope future firmware or OS updates will fix this annoying problem.Since the phone's keypad is not covered, just like any other uncovered keypad phones, when you're not using it, and storing it in your pants pocket (or purse or handbag), you should lock the keypad, by pressing and holding the "Talk" (blue phone icon) key. If you don't do it, it's very likely you will dial out inadvertently. The problem here is the phone's documentation doesn't mention about how to lock the phone, nor does it recommend it (while it should have). That's another negative which I hope HP will take note, and that's why this phone won't get 5 stars from me.