I figured out the same thing when deciding among carriers and phones. Sprint/Nextel owns both Boost Mobile and Virgin Mobile USA.
My wife and I both wanted an Android phone, and Virgin Mobile had the best one available on a prepaid plan.
Even with the upfront costs for the phone, we'll break-even on costs (Samsung Intercept would have been $99 instead of $219 if we'd have gone with a 2-year contract) after 10-months, and be well ahead of the game after 2 years. Another great thing is that I will be deployed for a good part of next year, and I'll be able to cancel my Virgin Mobile line (or go down to one of the $20/month plans where you never lose your payments in order to keep my number) and be able to reacctivate it or roll all the $20 payments into top-ups for my phone when I return. I can even use the phone wifi-only when I'm overseas as a sort-of ultra portable PC, I tested it on wifi only before I activated it and it worked fine without any service, if they can get the bugs out of the Skype app, it'll be even better.
We're both currently onthe $60/month unlimite everything plan from Virgin Mobile. The cheapest we'd of gotten with a family plan would have been $139 plus taxes and fees per month (so closer to $160). With Virgin Mobile it's $60 each plus about $4 in taxes, so about $128 per month for both of us. Now, if you have more than 2 phones in your houshold, then the contract plans where each phone is only incrementally more, typically $10 each line for voice/text and $40 for data, then the difference gets smaller (about even for 3 phones, then an advantage to contract plans for 4 phones).
Now, as far as quality of the connections goes, so far so good. We've had them for about 2 weeks now. Data is great. Using speedtest, I routinely get 500-900 kbps and have had a few spikes over 1 Mbps. Even though I have wifi at home, I usually keep it turned off becuase my 3 Mbps connection at home doesn't really offer much of an advantage for what I typically do with my intercept (though, if I may use it in the future for some home automation tasks I know use an iPod Touch for).
The only issues we've had so far are with MMS. They won't send at all if you're connected to a wifi network (which is another reason why I leave them turned off), there's typically a 10-15 minute delay between when they're sent and received between our two phones (not sure how long other carriers take to get them), and we've had 2-3 messages each that we've sent to eachother that never made it to the other. This is only with MMS, SMS comes and goes instantly. Searching the web shows that MMS issues seem to be widespread among all the carriers.
Bottom line, coming from having 2 feature phones on a T-mobile family plan and paying nearly $100 month for 750 minutes and unlimited text, we are extremely happy with the Virgin Mobile Intercepts for a little more than $20/month over our previous cost with unlimited everything.
