I've been on the Spint SERO plan for about 5 months.
Sprint's network is very extensive and I've never found an area where I'm without service. This isn't a problem anyways since you can roam for FREE on Verizon and Alltell's towers as well. Their data service is very fast and very reliable. It was also very widespread; I get EV-DO access everywhere I've been except in very rural parts of New Hampshire.
The phone choices aren't the best, but they do offer a few good phones. I bought the Motorola Q
The customer servce, unfortunately, is very poor. Pray that nothing is wrong with your bill because it will take hours to get it sorted out.
The best part though is the VALUE! For $30/mo I get 500 anytime minutes, unlimited nights and weekends starting at 7PM, unlimited mobile-to-mobile, unlimited data, and unlimited texts. The cheapest voice only plan from Cingular is $35!
Their online billing feature is very nice, though, and it's easy to pay your bill online.