At the very least, Amsoil is among the top quality products. I buy Amsoil because they provide a wealth of information, share their testing results, easy to email with technical questions, and I really enjoy knowing what I'm putting into my vehicles. I don't think anyone can go wrong with the right Amsoil product.
The Amsoil downside...price. Unless someone can show test results showing Amsoil performing poorly (within their prodcut specs, of course), I'll continue to respect Amsoil's products.