Short answer: it depends.
Longer answer: most people want reliable network coverage in a few specific places (their home, place of work, school, etc.).
If you travel a lot, particularly for business, you might want a network that performs acceptably wherever you go. If that includes international destinations, you will need to think how your phone's compatibility with the local cellular networks.
Verizon uses the CDMA technology which is not widely deployed beyond the U.S., South Korea, and Japan. Thus, most Verizon phones are useless when traveling abroad (there are a few "worldmode" phones that work with varying degrees of success in non-CDMA countries).
It's important to note that in the last Consumer Reports survey of U.S. mobile operators, none of them were good. Verizon ended up on top by a slim margin that was barely above the threshold of statistical significance. Basically, Verizon is a 37" giant in a land of three-foot midgets.
I'd love to get an iPhone, but I get zero reception on the AT&T network at home. Nothing. Would I buy a Verizon iPhone? Probably not. I still want a cellphone that works overseas. Thus, I am stuck waiting for the iPhone to arrive on T-Mobile (their signal is strong at home) or hoping that some day AT&T will place a cell tower closer to where I live. Who knows, maybe someone on the other side of my town has the opposite experience: AT&T is great and get zero bars with T-Mobile.
So really, it's up to you to assess network quality in the places you frequent the most and understand which one provides the best value. Then you have to think whether or not the functionality of any given handset (iPhone or other) is enough to justify the level of service offered by the carrier in question. If you move or change jobs, it's likely that you'll see some change in cellular reception quality with your existing carrier. Better or worse? You'll never know until it happens.
It's a value assessment that you will have to make yourself. As a matter of fact, you really wouldn't want anyone else to decide for you.
While I agree with most of the general concepts of your statements, there are some things with which I take issue. Assessing a carrier's network in your home area is a great idea. However, some of us travel all over the country, and the world. It's hard to predict what kind of service you'll get in new areas that you travel to.
I have had all three of the networks in question. I was an AT&T subscriber for over a decade when they were on their old TDMA network, left to go to Verizon for a year, came back to AT&T shortly after Cingular took them over, and now have a jailbroken iPhone on T-Mobile. Here's my view of them all:
AT&T's TDMA network could stand toe-to-toe with Verizon in most places, and their customer service was acceptable for me for the most part. But it could never stand up to today's demands. Besides, it's gone now, so that's all moot.
Verizon has, by far, the best and most saturated coverage in the 48 contiguous states. When I had a Verizon phone I had coverage in remote locations while others were sadly looking at no bars on their AT&T and T-Mobile devices. I was in Aspen recently, and while all of us who had T-Mobile experienced very spotty service and couldn't get a signal at all in the condo in the middle of town, the one Verizon user was able to offer up his Droid with 4 bars on 3G for use as a reliable tethering device for my laptop. But their customer service policies are not great, and they will always push their customers around as much as the market allows. I don't trust them. Case in point: I was recently in a Verizon store buying a Droid phone for a friend of mine for her birthday. The poor guy at the counter next to me was buying an upgrade from a straight phone to a new Blackberry. I overheard the Verizon guy tell him that he was going to charge him $20 to migrate his contacts over to the BB!! This is because of Verizon's technology, not anything the guy did. Total poor customer service proprietary BS. I wonder how many years ago Verizon paid for those transfer devices in the back of their stores...
I went back to AT&T after the Cingular merger because I needed a new phone, and Verizon's hardware offerings at the time were weak. Besides, I was tired of waiting 45 minutes for service every time I went into the store. What a nightmare that move was. AT&T/Cingular is a horrible, incompetent company. I left them before my contract was up, paid the ETF to get away, and will never give them another dime.
I have been on T-Mobile with both a BB for several years and an iPhone more recently. Fantastic company from a customer service/customer friendly standpoint. But their network lacks in saturation. Of course, using a jailbroken iPhone means no 3G for me. But my 3G BB is lightning fast. Interesting to note that T-Mobile is in the process of increasing both their 3G network speed (software updates). That update will cover 200 million people before the 4th quarter this year. They're also upgrading their POPs to 1Gb from 100Mb. These upgrades will make T-Mobile's 3G network faster than both AT&T and Verizon. In fact, my BB on the East Coast is already faster than AT&T.
So the Consumer Reports ratings, aside from the fact that CR tends to be very subjective in their rating system, are not all that helpful to a lot of users when you say that Verizon is "only marginally" better than the other carriers. Sure, when you take things like customer service, proprietary policies and lack of reliable international coverage into account they lose points. But for most US users, Verizon's network is going to be the 800 lb. gorilla that will make their phones just work. I, personally put a lot of weight into how my cellular company values and respects me as a customer. T-Mobile wins that hands down. But it does sometimes get frustrating to lose coverage and experience dead spots in major metropolitan areas. As far as AT&T is concerned, as I said they are a despicable company, and I am convinced they'd be in a lot of trouble if they hadn't gotten the iPhone. Hopefully T-Mobile and Verizon will get it soon, and AT&T will start to hemorrhage customers. A little karmic justice is in order.
