I have Verizon, and I only used it last year to message people who were at Roo so we could meet up. Verizon worked great, until my phone died. I could never find an available charging station that worked either. And we never left our cars running long enough to really get a good enough charge.
I had verizon and stayed in Centeroo the whole time ... maybe it's better in the campgrounds but it SUCKED! maybe it's cause i was in centeroo, maybe it's cause I have a smartphone that doesnt do as well but most people have them nowadays. I had the hardest time trying to reach friends. Texts were delayed for hours. Some calls wouldnt go through. it was frustrating. I do recommend planning ahead of time where to meet and what time. Also, many people recommend a dry erase board or notebook to write notes to each other at the campsite. Worse comes to worse and you really want to find a friend staying with you, go back to camp and see if they said what show they'd be at or a meeting place for later that day, etc.
We have had a lot of friends with us over the past several years carrying phones from all the major carriers and the only consistent pattern we found about cell service is that your best chance to use it without issues is in the mornings before everyone gets up. After that it is a total crapshoot as you may get lucky and have everything work fine or you may not be able to use your phone to send or recieve calls, texts, or internet at all.
I'm an employee of AT&T and I recently got an update on our network that there has been permanent towers installed/upgraded in Manchester. There will be "COWs" there as well (COWs are our mobile towers)
I'll still have my phone on Airplane mode because I'm taking it for the camera, note taking abilities, and light source.
These are very cool self contained mobile cell towers which are used to either temporarily boost capacity in an area for special events such as bonnaroo or as a replacement for a traditional tower when it is out of service.