As you are in the US, I assume you mean the US Virgin Mobile service, rather than the UK one that everyone appears to have already replied about.

In the US they share the Sprint network and appear to have reasonable coverage. We got my step daughter one for christmas and she doesn't appear to have a problem with not being able to make and receive calls. The only time she did struggle was when we were in Tahoe, but my Cingular and AT&T TDMA phones were struggling too.

For the money, and if their charging structure suits you then I think they are a good option.