I did get a bunch of warnings about roaming data and then text messages from verizon about running up a huge bill, but I called global services and they assured me it was a glitch and I would not be billed for anything but my plan. This happened to me once when I was using a blackberry too.
I had those warnings as well with the same response from VZW. No strange charges appeared on my bill, so all is well. The Global plan from VZW is so easy and seamless, I don't know why anyone would want to go through the hassle of getting a foreign sim and a new number unless they were going to make a lot of calls and/or stay for a long time. My experience with the Blackberry overseas was that the data and email were flawless. Traveling from one country to the next (12 in all), my BB would just give me an alert and say " Welcome to (insert country here)". No configuration at all. Voice calls could be a little pricey, but data was ~$2.50/day. There is no charge to use the service, just pay the pro-rated $64.99/month data fee. Only staying 15 days? Then charge would be ~$32.50. There may be some other charge to use the voice/text but it is minimal ($4.99/mo?) Don't get bogged down with the nickels and dimes...
I was hanging on to my Storm 1 because it works so well outside the US. This droid is great on my home network so I hope the experience will be the same as my BB when I go overseas!