When I had TMobile I was pleasnatly surprised at the low data charges, in the UK however phone calls were expensive. If you are tehering then this is more of a problem as there is a much lower limit on the number of Mb you can use and tehering to your computer quickly uses this up. I noticed similar charges in Singapore.
I have now changed to ATT, and my bill was higher when travelling in the UK due to phone calls. After noticing this I purchased a cheap cell phone for the calls in the uK and just used my blackberry for data and emails much better cost wise.
Now I travel to Canada a lot, and tend to use BBM to communicate with (sms messages are too expensive). I am careful with Tethering. (I have 50 Mb on the international plan, and have not noticed any high charges providing I do not exceed this number.