Except you're not stuck with using the phones provided by the carriers for prepaid...not in the slightest. With AT&T and T-Mobile you have GSM...same with their MVNOs. You can use any phone that uses a GSM Sim and supports that carrier's frequencies. Just get a sim from the carrier and setup and account with the carrier and you're good to go. I should know....I switched from Sprint post-paid to T-Mobile prepaid.
I'm using my Galaxy Nexus bought direct from Google in the summer. I'm on the $30/100 minute/unlimited text/5GB data plan. I don't go over the 100 minutes as I use VOIP at home and at work when on Wifi.
With that plan I save about $60 per month over what I was paying with Sprint (450 minute plan). In December I will have saved enough to offset the cost of the ETF. By June I will have saved enough to offset the cost of my GNex as well.
With Sprint, had I stuck with them, I wouldn't have been able to upgrade my phone until February...and that's with a new 2 year contract. Now I'm seriously considering upgrading for a second time to get Nexus 4.
Just depends on what you use with Prepaid...I'm saving tons.
Used this guide more or less..