You've no doubt heard the horror stories from people who've brought their smartphones with them to another country and were surprised with a roaming bill for thousands of dollars.
I have recurring dreams of it happening to me. When I travel overseas, I bring along a paperclip to remove my SIM card from my iPhone when I'm on the plane so that when I land in another country I don't get charged AT&T's exorbitant international roaming fees. Call me paranoid, but I have read reports that people get hit with roaming charges even after they turn the roaming setting "off" on their phones. By removing the SIM card (which is easy) I'm ensured that I won't get an unexpected fee.
On my last few trips abroad, I've purchased data SIM cards ahead of time. I've had great experiences with B-Mobile for Japan and HolidayPhone for Italy. I had a terribly frustrating experience with RebelFone (Here's my post about b-mobile and RebelFone).
So I read with interest about a new company called KnowRoaming, which sells a little sticker that you can apply to your phone's existing SIM card. The company says you can save up to 85% off of your voice and data in 220 countries. It sounds cool -- I would never have to buy another third-party Sim card again. But then I got to the bottom of the page and checked out the fees.
If I use KnowRoaming in Japan, I will be charged $.42 per megabyte. That's $420 per gigabyte. With the amount of data I use when I travel internationally (Skype, constant use of maps, restaurant recommendation services, downloading important attachments from my email, etc.) I could easily burn through 1GB in a few days. The b-mobile SIM card with 1GB of data costs ￥3,980, or about $40. So it's less than 1/10th the price, at least at my level of data consumption.
I paid $80 for the HolidayPhone SIM card, which had 10 GB (!) and also free incoming calls to my regular phone number. KnowRoaming charges $.14 a minute for incoming calls and $.13 per megabyte for data. I was in Italy for a week, and I would conservatively say I used 3 GB of data while I was there. KnowRoaming would have charged $390 for the data alone. I used the phone for incoming calls for about two and a half hours. That's another $20, so I'd have been looking at a bill of over $400 from KnowRoaming.
Unless KnowRoaming drastically reduces its rates, I'm going to stick with HolidayPhone, which has service in almost every country I would ever need to visit. (I'm also going to check out this other service called Roamer, which looks promising.)