Get Everything Done

Like many people here, I've gone through many trials of systems and many iterations of AF. Some work form me, some don't. Here's how I test a system:
1. I Read and understand the directions and principles underlying it as best and most thoroughly as I can, asking questions about it on this forum if I need to;
2. I start the system, in whatever way the instructions suggest to start it. (For example, instructions might suggest using a fresh list, using a shorter list (like FVP), using whatever I was already using);
3. I Use it as my main task management system "for a time." How much time? Long enough for me to understand the system and before my life turns to chaos if the system doesn't work. (see 4);
3. I Check back with the instructions, making sure I'm doing it correctly, especially if I get into trouble. What kind of trouble? Usually a missed task or a feeling of panic because something is missing.
4. I pay very close attention to my feelings. I find that when a system is not working, after a few days with it, or sometimes as long as a week or so, my life starts feeling out of control. This, to me, is the main reason for abandoning a system - I'm looking for David Allen's "mind like water." I realize that no system can create that sense of peace, but a poorly functioning system can take it away.
5. If the system is not working, I go back to whatever was working previously, which becomes a kind of benchmark system for me. Then, and only then will I start thinking about ways of tweaking the new system, all the while keeping up with my stuff because I'm using a system that works. For example, if this system eventually fails me (as FV had done in the past) I'll just go back to the random method.
I will only add what we all know, that this is all very personal and relative. All I can say at this point is that if FV or FVP doesn't work for me, it doesn't mean its a bad system, just that it doesn't fit well with my temperament. On the contrary, all of Mark's systems have been fantastic, the benchmarks I have used have been really outstanding and set a very high standard. As a working system, nothing comes close the the effectiveness of AF. I'd be interested in a deeper understanding of why it works so well.