I've never used it, but from everything I've seen Amsoil is an excellent oil. However, because most people don't take advantage of the extended drain intervals, I don't think that it makes sense financially.
If you were going to use a more affordable quality synthetic such as QSUD, and run it for a reasonable OIC (obviously the term "reasonable" varies by the vehicle, driving style, commute type, etc), I believe your vehicle would last just as long as if you had used Amsoil.
Engine oils and vehicle engineering have advanced so much, that as long as you're not running it past the abilities of the oil, it's very very unlikely that you'll ever have an engine problem caused by the lack of the oil's ability to protect.
But if you can get Amsoil at a decent price, and run it for extended drains, then it can definitely become financially worth it. I know I'm planning to get some Amsoil gear oil to put in my differentials the next time I do a drain and fill because I don't know when I'll get a chance to change it again, and I believe in Amsoil's ability to last for a long time.