This is really up to interpretation so everyone is right in one way or another.
Here is my take on the subject: Someone who is truly successful with SPS probably should have a handful of large, colorful, healthy sps corals that are at least a few years old.
This means they have sustained their system without introducing pests, have kept the corals alive long enough to grow into colonies, not overcrowded their tank, and maintained good parameters.
This reminds me of a similar discussion brought to one of our meetings by Anthony Calfo. He basically stated that he can't consider himself successful keeping certain fish, even though he has had them for 8+ years, because in the wild they live for 20+. I'm not sure that this applies here because corals can live for a REALLY long time in the wild....so success relative to what?
I've been keeping SPS for around 3 years now. I have a few very large, colorful, healthy colonies that I have had since then. I think I am finally getting to the point where I can avoid all the stupid mistakes (because I've made them all already). Having said that, from here out I would consider myself keeping sps successfully.