Here's a bit of insight into my selection process. While I currently use AMSOIL, I've got more hours, miles, and oil changes with M1 products.
As I understand it (feel free to correct me), there are only a couple of manufacturers of synthetic base stocks. Mobil is the largest, and developed their technology with lots of DoD money (read: 'our parent's money' for many of us). I think that that Petro Canada makes some of the base stocks that AMSOIL uses as well.
There are other additive package manufacturers besides Lubrizol, but they're the biggie.
AMSOIL goes into the selection and blending process with a different philosophy. They build their products for the long haul. You might not agree with the 25K/1 year interval for most of their oils, but they do, and have from the beginning. And where the formulations have fallen short, they've worked to improve them. Many customers have no trouble making 25K. Many run 150K with the same oils. I've seen their products do that - this isn't a case of 'a friend's brother's nephew who sells AMSOIL said that a client's brother...' thing.
Contrast the path Mobil choose to take with M1. It was also a 25K/1 year oil. There were acceptance probs here, and probably a couple of other problems as well. Instead of sticking with that goal and improving the product first, cost second, the decision was made to lower the change interval and formulate the product for a 7500 mile/1 year change. There are folks that have successfully gone beyond this with M1 products but not nearly as many. Before finding this site, I only knew of 3 or 4.
I don't consider this to be an attack or a validation of either product.
In the general aviation market there are older style airplanes with fabric covered wings that carry 2 people almost 100mph for just over 2 hours. And there are 6 passenger jets that can fly high and fast and from coast to coast. Each is better than the other for a specific function - the Taylorcraft does what it does very well, and so does the Learjet.
I like very much that I don't have to change the oil every couple of months. I like that I'm not adding the additional 27 quarts of drain oil to the hazardous waste process. And I like very much that I can purchase these products for less than I can purchase other synths. And I still kinda giggle when I get a tax refund on my oil changes besides.
I also completely agree that there are some (many?) waaaay over the top AMSOIL salespeople out there. As much as I'd like to start muzzling some of them, and deleting lots of web page content, no one died and made me the sales censor. In typical free-market fashion, I'll not support those folks that I don't feel comfortable with or agree with. And I also recognize (more and more as time goes on) that my ignorance will make some companies' products look 'too good to be true' and I'll miss a good opportunity. ((I got an AMSOIL catalog from a gent in Colorado about 5 years ago. After waving the BS flag around the living room, I tossed the catalog in the wood stove.)) But I can separate the independent dealer's religious zeal from the company and their products.
Just my $.02.
Andy
PS... FWIW: Ya'll can 'blame' my presence here on a waaaaay over the top Schaeffers salesman that was badmouthing full synth oils on another forum I frequent.... He just could not 'get' that I MUST use a full synth in my car. He was so hung on the benefits of Mo that he couldn't hear that semi synth has lots of petroleum in it.