I'm not sure facination is the word. For me it is a little loyalty, but based on my experiences with it.
Starting with my motorcycle road racing (mid-late 80's), I noted immediately better shifting and about 2.5mph faster top end on the track. After a season of total racing abuse we tore that engine down. It was like new. Cross hatch looked perfect, no deposites on the valves, the cams and rod/crank bearing looked new. Keep in mind most guys with regular oil were changing there oil every race day, I went 3 race days on each change of Amsoil.
I also started using it in my vehicles. I still have my 1987 Nissan "hard body" 4x4 pick up and still going strong with 245K on it... and again, extended oil drains (7000-8,000 mile intervals).
My girlfriend had a Chevy S-10 pick up with a transmission that was really hard to get into first and second gears, so bad we thought we were going to have to have it re-built... after going Amsoil synthetic it was almost un-noticable.
SO... I guess for me every experience I've had with Amsoil has been positive. I'm not saying that other synthetic's aren't good as well. I do firmly believe synthetics are superior to petrolium oils.
That is why I personally stick with (Amsoil) synthetic oils. I suppose if I had started with Mobile One or another syn brand I'd still be using that... but Amsoil has all the synthetic oils I need, not just one.
My 2 cents, and experience...