It's rubbish.
My niece purchased an iPhone 4S - sim free and unlocked to all networks.
Inserted her Virgin Mobile sim (a rolling 1 month deal) to find that 3G did not work - no Internet browsing whatsoever.
Phoned virgin who sent her a text with the settings required. Installed those and still got nothing.
The next day it suddenly starting working by itself. A day later it stopped again.
This time I rang Virgin on her behalf and they told me that they do not support the iPhone. The customer service rep actually said that he used an iPhone himself with no issues, but couldn't give me any more advice since Virgin do not support them!
After some arguing I managed to get him to go through his personal phone so that I could replicate the settings. Dd this, but still no reliable 3G.
In the end she disconnected and went with another provider, where her iPhone worked fine straight away.