You well know that asking these questions in a forum where the predominant membership is composed of users who have experienced problems with the device, that you are going to get predominantly negative comments which may well be far from an accurate reflection of the user experience of the iPhone as a whole. If you're looking for a self-fulfilling prophecy, this is a good way to go about it!
I've had a significant number of cellphones over the years and they have all been pretty much junk. The last one I had was a Moto RAZR V3m, and I can safely say that if it were the only cellphone on the market, I wouldn't have ever bothered to have one. It routinely missed calls, dropped calls, produced poor, echoing audio, suffered sudden battery failures and was tediously fiddly and obstructive to use. By comparison the iPhone has been a breath of fresh air, not least because it's proven to be entirely trustworthy - and that despite having swapped from US Cellular, which had excellent coverage, to AT&T which does not.
All in all though, however clever, smart or versatile a cellphone, the one thing it must do right is make and take phone calls, and if you are in any doubt about the iPhone's capabilities in this regard, it is simply not a wise choice to make to get one. Certainly it may prove itself excellent in service, but as an unknown quantity in the context of your personal needs and patterns of use, it's a risk you make clear you can't actually take. Better to forgo the niceties of design and instead opt for something you already know to be functional in your own personal context.
For the sake of completeness:
a) May not be among the best in weak signal areas, possibly because of the internal antenna, the inclusion of so much other circuitry, or whatever.
In comparison to my old AT&T Blackberry, the iPhone signal is almost identical. It occasionally dips lower in the interior of buildings than the Blackberry, and varies a little more, but typically I've not found the AT&T signal to be all that good amongst buildings anyway. There is no doubt that where a less than optimal signal is concerned, a device with external antenna is generally going to outperform one with an internal one - the human body is a rather effective 'signal damper' at these frequencies.
b) May lock up or otherwise misbehave because of corruption of the software, the general complexity of the device, etc. -- especially anything that requires a computer to fix, runs down the battery, or makes it impossible to use the touch screen to place calls.
While there are always examples of units which do suffer such problems, simply because manufacturing is never 100%, my experience of the iPhone has been that it is very stable as a platform. It has behaved predictably in the almost 3 months I have had one, and not caused any concerns in terms of whether or not it was functioning correctly. I would expect that despite the complexity, the majority of users as a whole would likely say the same thing. When you consider the furore here at the price cut or the 1.1.1 update, and set that against the general complaints and failure reports here, you'll get the distinct impression that while there are unhappy owners, there aren't an avalanche of them complaining about the same things.
c) Has an non-user-relaceable battery so I can't use a spare as a backup or replacement should the provided one run down or lose recharge capacity from (for example) accidentally leaving the phone in a freezing or overheated car.
The internal battery has always been used as a stick to beat Apple, not just with the iPhone but iPods previously. Oddly though, that hasn't prevented these devices selling well and being very widely used. It certainly limits the option of carrying a spare, though it is also of note that carrying a spare in terms of this battery technology is not always such a good idea, given the self-discharge rates. Basically while the capability to swap-out batteries would give some users a benefit, it also then creates a more complex pattern of use. The iPhone (rightly or wrongly) is designed to simplify, not complicate.
In terms of trustworthiness - I don't know of any carrier or device in the US that I would trust as much more than a toy. The cellular market here is too heavily dominated by stagnant players with vested interests in not doing anything new. By comparison to Europe, where cellphone services have been far more robust and carriers far more reactive, service quality and standards are a joke and the market is really quite immature. Emphasis is placed on gimmicks and flourishing feature sets, not on quality products that do what they actually need to do. As a result, I'd say if your existing 4-year old device does what you have needed it to do, the lack of the bells and whistles on it is really part of the reason, and is largely the reason you should keep that and continue to use it.
For me, I have thoroughly disliked the cellphones I have had in recent years. Cheap, nasty little things, designed to appeal by gimmickry alone. That's actually one of the reasons I like the iPhone, because as much as it is a toy, it doesn't pack all those gimmicks or pretend to substitute for them. It is, instead, a simple, fairly unexciting, communications tool, with an interface that makes it usable.As such, in my pattern of usage and needs, it fits much better than anything else so far, and gives me no reason to trust it less than anything I've had previously. Even then, however, I think there is a long road ahead in the development of it before those who need to take it seriously will be able to really do so.