No.
I believe Amsoil makes some fine products, but I also believe other leading motor oils are also fine products. As others have mentioned, there are several real disadvantages to Amsoil.
First is limited availability.
Second is closely related to the first, but it is different, and that I totally don't like the marketing/distribution/retail (or whatever you want to call it) system. I know that someone will say that I can become a distributor. Why should I have to, just to buy motor oil?
Third, price. With the rebate that Mobil is offering, I can get a 5 quart jug of M1 for $25, even if I buy it at Autozone, which isn't one of the less expensive sources for motor oil, or for $17 at NAPA right now. What will 5 quarts of XL cost me? $44 + shipping from Amsoil. I know that someone is going to tell me that I can get it cheaper from a distributor, or set myself up as a distributor. But I shouldn't have to! Just give me a fair price!! Which brings me to:
Fourth, pricing structure. I don't want to pay an unfair price, but I don't want to track down a distributor!
The only advantage I can see to Amsoil is if you are willing to do 25k mile oil change intervals with their Signature product. But if you are going to put Signature in your engine and then get cold feet and change oil at 15k, then you just as well have saved a bundle and went with M1EP.
I've seen claims by some about how clean Amsoil keeps an engine. But I don't think that Amsoil is really any better than any other leading brand motor oil. Plus, there is no evidence that anyone has been able to give, that shows an engine on Amsoil will outlast, outperform, or have better economy, than an engine on any other leading synthetic.
There are plenty of documented cases of cars going well over 1,000,000 miles, including that guy that has driven over 3,000,000 miles in his Volvo, and a BMW that went over 1,000,000 miles on Mobil 1. Is Amsoil better than that?
If Amsoil was available through normal retail outlets for a competitive price, it would be in my oil stash alongside the Castrol Edge, Pennzoil Platinum, Mobil 1, and Royal Purple (that I picked up for $3/qt on closeout at Walmart). Until then, I'll drive my cars as long as I can stand to see them, with other leading brand motor oils in the crankcase.