I say that most of what he says, and what many other GPS detractors say, is wrong because if even a small part of what they say is right, there would be no way that me and several other people who have carefully tested the devices could get anywhere close to the accuracy we are seeing.
In the past I have gone through their arguments point by point and explained why they are wrong or irrelevant, and then a week later someone posts the same or similar article, so at some point I say why bother. But you sound like someone who is genuinely interested, so here goes.
His first argument is that the manufacturers themselves give the accuracy of their devices to be only within 8 meters, or whatever. The accuracy of the GPS device in determining the absolute position of a single point is irrelevant in determining distance traveled. What is relevant is what portion of that error is systematic and what portion is random. It is not uncommon when you download a track to see all the points shifted a couple meters to the east. This is systematic error and it has no effect on the distance calculation. What does matter is the random error, one point is to the left of you and the next is to the right of you, and all evidence suggests that it is pretty small, certainly no where near 8 meters.
His collie analogy is ridiculous. Anyone who has ever downloaded a track from their GPS knows that you never see one point several meters to to your left and the next point several meters to your right, which is what he suggests with his collie analogy. You don't see points way inside the infield and "up in the bleachers." And again, if this actually happened and that is the only info they used to calculate distance, you'd see errors in the 5-10% range, which no one does.
All of this goes out the window when there are tall buildings around. In that case you will see crazy tracks and bad distance info. But he is suggesting it happens all the time.
His statements that the GPS units calculate distance by simply "connecting the dots" and that the GPS units take measurements every 1 second or 5 seconds, are things that he doesn't know. The units may STORE points only every 1 second, but for all we know they are taking measurements every 10 milliseconds and using those to help calculate distance. And they may be applying real-time filtering to the points as part of the process of calculating the distance, not just connecting the dots. We just don't know. To state these two things as limitations of how accurate the units can be is wrong.
If someone is going to write an article about the accuracy of GPS units, they should start with the empirical evidence and fit their theories to that. Most, if not all, of these articles are written by people who have obviously never actually tested the devices.
The part of his article about the measurement process is very good. The part on why GPS can't be accurate is not.
BTW, while most of my tests were done with my bike-mounted GPS, several of the tests were with a Forerunner, some while riding and some while running. There Forerunner did appear to be slightly less accurate, but on the order of 0.1-0.2% less accurate.