I have been using their Signature Series Oil for over a decade now and you can see the miles I got out of my Santa Fe in my signature. This spent most of its life on Amsoil SSO 0w30. When the engine died of a broken camshaft it wasn't using any oil or leaking any oil. It's too bad it died or I would still be driving it.
I'm now using the SSO 5w20 in my Dodge Journey and I have sampled the oil last year and done a UOA which showed almost all 0's for wear metals and the additive / TBN was virtually untouched after 5,000km. I have a sample sent into the lab now and I'm just waiting for the results. It was sampled at 10,000km. I typically run my oil changes out to 10,000 - 15,000km depending on the type of driving I did over that OCI.
Once all my Amsoil filters are used up that I have in my stash I will be going with Amsoil bypass filtration and pushing the oil as far as it will let me. I will have lab tests to back all this up.
Their newest formulation change promises 100% protection from LSPI and even more cleaning power than before. I'm not sure how the oils could get any better but if it's true then I can't wait to see the UOA's from that.
The reason I use it here in Canada is because the pricing I get with my preferred customer membership, their points discount program, and various sales they run makes it less than the cost of other brands off the shelf like M1, Pennzoil etc. this combined with the excellent performance I have seen first hand makes it a no brainier and it is why I have stuck with the brand.