I have that plan enabled on occasion on my BB8830. It gets GPRS speeds, not 3G, in GSM locations and isn't too terrible. Coverage can vary significantly depending on the device. I had no coverage in Japan (not a huge surprise) but also not in Panama on the 8830. The Storm offers coverage in both those countries but has other issues.

You really should activate it in the US first as they need to enable the SIM and then you have to update the roaming settings on the device. It takes about 5 minutes to go through that activation so if your friend likes you enough they should be able to do that for you.

I have never had a "hidden" charge crop up. It only covers data, not SMS or voice, so those can ring up charges pretty quickly, but otherwise everything has been above board on the billing side.