Boost is iDEN and has identical coverage to Nextel.
Virgin is CDMA and uses Sprint towers, different technology.
My Sprint phone also roams on Verizon or Alltel towers when I am outside their service area (rural areas like SW Georgia) but I'm not sure whether Virgin would roam through the same providers or not.
Nextel/Boost is the only iDEN network in the USA, so they would not have partners to roam through. They only work on Nextel towers, and therefore you need to make sure Nextel covers the area you plan to use the phone.
They really are apples and oranges, Nextel is great if you stay in the city, and it's even better if you can 2-way your friends for free all the time. If you travel and want the most coverage in the most places, I recommend a CDMA network. Hope this helps.
