The only significant differences are in monthly rates ($35-45-55 on VM, $60 and up on AT&T or Sprint), and the lack of 4G. The monthly plan difference is significant, since VM's plans have unlimited data and text (which for many users are fairly essential), and on other carriers, they are essentially add-on services. As for 4G, outside of a fairly small metropolitan areas, it is still not accessible from vast swaths of the US. Sprint's 3G service is fairly decent (up to 1.8Mbps in some places), and has a fairly large coverate.
My subjective assessment is that the VM offering is much better, all things considered, than others.
Virgin Mobile runs on Sprint's towers so if you get good service with Sprint in your area, then you'll get good service with Virgin Mobile. The biggest difference is that you pay more up front and less in the long run.
answered 2 years, 2 months ago