I have Virgin, I am a light user and I recommend this as a standout in terms of affordability. The only real gotcha is that you have to buy a phone up front, but the rave is only $60 and comes with $10 or $20 in free airtime to start (depending on where you buy and what deal is in effect at the time) and considering what I am saving on a monthly basis, the phone pays for itsself in a few months. The rave also has GPS location for 911 calls, a 3mm LED flashlight, voice recognition dialing and overall is a decent phone.
Sprint PCS may not be the best network in terms of clarity and maybe even coverage, depending on your location, but I have found it to be completely adequate.
There is no problem buying more minutes right from the cell phone with a registered credit card (there are other methods, too) and the munutes always roll over as long as you do it within 60 days of your expiration date. Yes: you get 90 days active and then 60 days inactive before you lose minutes, and even then you can transfer them to another phone if you want to. I figure my $20 every 90 days gets me roughly 30 minutes per month, and with unlimited rollover, I am accumulating minutes rather than running out. Between that and the discounted rate after 10 minutes in one day, I am never afraid of getting stuck on a long call or ocassionally needing it for heavy use. My wife and I both have VM phones and are both happy. She is a lot gabbier than I am, but she still normally gets about 2 months out of her $20, so it is still the most affordable option for her by a large margin.
We had one problem with one phone and VM replaced it for free, but dealing with the peppy but incompetent customer service people was unbelievably frustrating.
I recommend buying the phone directly from the virginmobileusa.com website. The cost is the same, overnight shipping is free and I think you get the most free minutes this way. Get an aftermarket case rather than the VM ones. That'll run an extra $10-20.
You also get unlimited free incoming paging and text messaging and voice mail and net alerts. Outgoing text messages are 10 cents each. You never have to worry about roaming or long distance because it is always the same rate. International calls are more, of course. Billing is by the whole minute and starts when you press "send" which is standard in the industry. This is a well rounded plan.
For $6.67/month, you can't come close to beating this deal for the light user. If you can control your usage and don't want to be on a cell phone all the time, this can save you a fortune over the years. But if you are using hundreds of minutes per month, this is probably going to be more expensive than a monthly plan. And if you are using thousands of minutes per month, this plan will kill you.
By the way, remember that you can now usually keep your current cell phone number when switching to a new carrier, but you have to follow the correct procedure. Contact your current carrier to find out how.