I've seen a few of these around and they look decent enough. But a few technical issues should probably be raised, if we are going to compare the images on these posters and billboards to those of other cameras.
Firstly, the resolution of an iPhone, (8 Megapixels I believe), is no different to any other camera which has an 8MP resolution - for example my Canon EOS 350. In fact, even the Leica 8 at 10MP, isnt significantly better. So, in that regards, resolution has no more or less bearing on the print quality of the image.
In relation to this, billboards are printed with an extremely course halftone screen - that's the large dots that you can see when you are up close to them.
This page (about a third of the way down) lists some viewing distances and relative halftone Line Screen resolution requirements (not to be confused
with dots per inch, or pixels per inch – which is what we mean when we talk about camera megapixels).
At those low line screen requirements, 8 megapixels might well be plenty for a poster or billboard (depending on the size), designed to be viewed at a distance. In fact, because the quality of a halftone printed image only increases up to a maximum of 2.5 times the halftone Line Screen, there may be no resolution advantage in using a 50MP dSLR over an iPhone, in many cases.
As for JPEG artefacts. Again, the iPhone JPEGs should not be significantly different in quality to those of any other 'out of camera' JPEGs. And there are several Apps that allow you to shoot uncompressed images on an iPhone.
Obviously, the iPhone cannot compare in many other areas. You can only do so much with a lens in a device that thin. Similarly, with the sensor size, dynamic range (which is where I would say I, subjectively, noticed quality issues) and so on.
As I said, the above relates to this disussion regarding commercially printed billboards. There may be some different factors involved in prints from inkjet printers, or whatever.