I've had Virgin Mobile for several years and it's been fine...I even get a signal in the back bowl at Arapahoe Basin (where no other networks get squat).
My Dad picked up the I2 for free a few months back by signing up for a two year plan. A bargain if you are expecting to stick with Verizon anyway. The monthly plans are much cheaper but I wouldn't trust their networks. My brother is dropping AT&T due to dropped calls and Sprint is the worst.