Virgin Mobile is a subsidiary company of Sprint and therefore uses Sprint's CDMA network. However, while Sprint did expand Virgin Mobile's network quite a bit after their acquisition in 2009, the Sprint network is behind competitors quite a bit and leaves something to be lacking for their MVNOs.
Currently, Virgin Mobile's network is mostly restricted to urban areas, and if you look on a map you can almost see the spidery lines between cities where Virgin has service. The company has great coverage in the Great Lakes areas, parts of the east coast and much of Florida. That being said, there are also several areas that have absolutely no service such as Wyoming, Montana, South Dakota and most of North Dakota. If you do happen to have Virgin Mobile coverage in your area, then rejoice, because virtually all of their limited coverage area gets 4G speeds.
Virgin Mobile offers both 3G and 4G speeds to their customers, and the expected download speed for the 4G network can be anywhere from 3 - 6 mbps, making their network good for streaming lower-quality video from sites like YouTube.
Until Sprint upgrades their network further, however, if you are a traveler then you might not want to use Virgin as you might find yourself stranded with no phone service pretty frequently. One good side is that Virgin Mobile does allow for roaming much like Sprint postpaid customers, although users can expect slower speeds.
I have read plenty of reports that the Virgin Mobile coverage map is a little inaccurate, so be aware that the map isn't updated very frequently and should be used as a guideline only. You might want to test out the service before you port over an important number to ensure the best service. To take a look at Virgin Mobile's coverage map, click here.