I used T-Mobile for awhile, and it was OK; customer service was decent but I got terrible reception everywhere (particularly my house). When Stynx and I went to one plan, I joined him on Verizon and reception improved dramatically.

I was with Cingular and stayed with them until they were assimilated by AT&T. The service (customer and phone) got progressively worse. When the contract was up, switched to Verizon. Customer service is somewhat better, but the phone service is way better.

T-Mobile here... I get great reception in Atlanta, less so out in small towns. danf, I'd talk to some of your local friends about this because reception in your area is probably the biggest factor in whether you'll be happy with a carrier or not.

Verizon purely because they get the best coverage for me. So, not really applicable to your situation.

I've had Verizon service longer than there's been a Verizon and have generally been a happy customer. My wife has T-Mobile (via her employer) and I generally have much better service than she does.

Verizon because I live in a bizarre part of NJ that doesn't get anything resembling adequate coverage from any other provider. I'm pretty pleased with it, all things considered. It's expensive, but the coverage is amazing and I haven't dropped a call in years.
FWIW, CREDO (like most MVNOs in the US) uses the Sprint network. So if Sprint coverage sucks in your area like it does in mine, you won't want to use them. That said, I know a couple of very happy CREDO customers, but they both live in urban centers where Sprint has solid coverage.
If you wanted to try a prepaid service, PagePlus uses the Verizon network--I believe they are the only MVNO on said network. They're known for being very flexible about activating phones from Verizon or Sprint (by which I mean eBay). They don't have anything like CREDO's political features, though. I can't vouch for it myself, but it does seem like a low-risk proposition to test it out.

I use Tracfone. If you don't want a smartphone, Tracfone are a brilliant deal. They contract with your local provider (Congular, T-Mobile, whoever) to provide local service. Normally, I pay $99 + tax/year for 800 minutes (I paid an extra $20 for a double-minutes-for-the-lifetime-of-your-phone card, a few years ago - most newer phones come with double minutes). That's over an hour of talk-time for $9/month (including tax).
If you use a lot of minutes, they have family plans. But the pay-as-you-go is wonderful. They roll over minutes from year to year -- I now have way more than I can possibly use and was able to take advantage of the $149.99 for two years plan, last year .. :-)
I have always obtained a connection, even in the wilderness (frequently lending my phone to people with classy plans, who can't get a signal). Only once in 10 years have I had a connection drop. I really rate their service.

I have Verizon and my husband has a Tracphone. We are both happy. Verizon is more expensive, but service is available in the most inaccessible areas.

I use Optus and have done so since I got my first mobile phone in the late '90s (before they were Optus) and have seen no reason to change. Hey, at least they're not Telstra, even though they may be only marginally less evil. Every time I get a new phone (always as early as my contract allows an upgrade), I look around but Optus always seems to be the best fit price-wise for my needs. Also, the reception at home is slightly better than other carriers (ie marginal reception compared to none). My partner uses 3 (now owned by Vodaphone), as does one of my daughters and they're happy with what they get, although frustrated by the lack of signal at home. Oddly enough, 3 uses Telstra's network and, although they can only get a signal in certain places at home, my (work provided) iPad uses Telstra and gets some signal pretty much anywhere at home - better antenna, I guess.

I've always been happy with the service from Optus, but my view could be skewed because I have an 'account manager' assigned to me as a legacy of some years ago when we were spending ~300 a month, making us eligible for this service and they've never taken it away. It means that, if I need something, I send an SMS to a number and a human (the same one each time) calls me with all my details ready and deals with whatever I need.

So I'm the only Sprint user? I am only loyal to them because I got in when the SERO plan opened to the public for about 10 minutes six years ago. 500 mins a month, unlimited texts, free mobile to mobile, unlimited data plan, picture messaging, and more. I live in an urban area, great coverage. Sprint customer service is spotty, but whose isn't?

I was with Cingular/AT&T for ages because I liked the kinds of free phones they had offered and have almost never been an early tech adopter. However, with the previous job, I needed a phone that could connect with the office Microsoft Exchange and access my work email and the Pantech phones they had were kinda crappy. I figured it was time to join the smartphone era, and when we learned that the company had discounts with Verizon, I went with them because their coverage in NYC was better way better than AT&T and they had Droid models I liked.

Even after I left the company, my bill with them is still lower than what it was with AT&T for the most basic service with an unlimited smartphone data plan. (In fact, when I contacted them by email to let them know I wasn't eligible for the company discount anymore, they adjusted my service and said I didn't have to pay the back-discount. Isn't that nice?)

Verizon. I started there, and I liked them. Then I switched to AT&T, which I found dreadful, due to poor coverage, high bills, and increasingly dreadful customer service. I went back to Verizon about two years ago and am very happy. They have excellent coverage, reasonable rates, and consistently good and helpful customer service. That's all I need.

The folks I know who use Credo are fairly happy with it, I think because they have reasonable expectations. It's just the Sprint network, so it'll be neither very good nor very bad in most places. The phone selection doesn't include a lot of the latest and greatest. And they're a cell-phone company, not a hippie commune or an anarchist infoshop or whatever--less evil than AT&T or Verizon, certainly, but a for-profit corporation that's going to ignore some issues and take some relatively-moderate positions on others.

If I was picking a new cellphone company today, I'd seriously consider them (especially if I wasn't into smartphones).