It's not as many countries, but with 3 you can get unlimited data at a fraction of the cost (I currently pay £15/month with 3 for unlimited data, 5000 3 to 3 minutes, 5000 texts and 2000 minutes to other networks).
Likewise, although I'm finding that Telstra throttles me quite a lot. Turns out you can use any network in Australia (with no roaming fee from 3), so I've manually switched to Optus and my speed has dramatically increased.
http://www.three.co.uk/Discover/Phones/Feel_At_Home
It's not as many countries, but with 3 you can get unlimited data at a fraction of the cost (I currently pay £15/month with 3 for unlimited data, 5000 3 to 3 minutes, 5000 texts and 2000 minutes to other networks).