Well, 1MB = $19. So check how much data you use on a normal basis, and you'll probably exceed $60/day. There are also so many processes that run in the background, weather, news, updates and more - so if you are not very, very careful, you'll pull 10MB in a matter of minutes. I'm a heavy user, and I got well over 5GB/month.

Those people that end up with $4000 bills are not all morons - they just underestimate how quickly data is moved around.

Bottom line, unless someone else is paying the bills, get it unlocked and go prepaid, or disable data roaming and stick to Wi-Fi.