Recently I had to get my own mobile phone service and decided to forgo the standard post-paid cell service and go prepaid. Decision was largely cost based since I already had my own GSM phone and planned to buy a Nexus 4. I did quite a bit of research and ended up with Straight Talk service
Straight Talk is a MVNO (Mobile virtual network operator) that leases network capacity from T-Mobile USA and AT&T. To sign up you either order a SIM from their web site or you can pick up starter package at Walmart. I did the latter option. In the package they provided me with 2 different mini-SIM cards and a micro-SIM card. SIM cards are really T-Mobile and AT&T unbranded SIM cards. Pick the card that is supported by the phone e.g. if it's a locked phone like AT&T use the AT&T SIM, change some of the phone settings (APN) and off you go. Quality of the signal is the same as if you used T-Mobile or AT&T. I picked unlimited everything plan for $45/month with a T-Mobile SIM. If you sign up for auto refill they cut it down to $42.50. Drawback is lack of international roaming and iffy customer service ie. hold times can be 30-40 minutes.
Another option I considered was T-Mobile's prepaid service called Go Smart which is similarly priced
I decided against it since cost was similar but with Straight Talk I have the option to switch to AT&T if I ever find T-Mobile coverage inadequate. That said Go Smart does have a wider array of calling plans so it may still be a good choice.
While we are at it I can also recommend an inexpensive VoIP service called GalaxyVoice. I use their free-tier which gives you up to 60 minutes of outgoing calls a month and all I pay is for taxes and 911 compliance ~ $3/month. You just need to pay the signup cost of $25 and get your own SIP device.
An extra bonus is that their web site is fairly unsophisticated and easy to automate for certain things e.g. forwarding my home phone calls to my cell phone