Lots of good questions.
First, people call the band a "Tool". I personally do not understand this concept because a tool is something that makes work easier. Being banded has not been work for me. Well, maybe yes in the early stages.
To me, the band was "surgical intervention" that, were diets have failed me in the past, this is a foreign object wrapped around my stomach and adjusted (tightened) that prevents me from doing something that I tend to do.
And that is overeating. Period. Simple.
The 2nd thing the band does for me, is it stimulates certain nerves signaling the brain that I am not hungry. Notice I did not say "Full". There is a difference between the 2.
Example: Yesterday was thanksgiving. I sat at the table and ate till I felt full. Which was very early, eating very little because of the band and the pouch.
Today, as I am typing this, I have not had a single thing to eat yet today (other than coffee and some water) and I am not hungry. I even went to the gym this am, so I should be hungry enough to eat a very large Breakfast. But I don't feel like eating. That's the band talking to my brain! Also keep in mind it took me 4 months past surgery to start to get these results. It was not instant.
So yes, as you say, diets have failed before. They failed because WE have to do the work with discipline. That is why I needed to have my stomach banded! Because I could not do it.
Also, keep in mind as you read all the posts on this board.
As with any "Bell Curve" you will have people on both ends. Those who have had the surgery and never need a adjustment (fill) and loose weight just fine, and at the other end there are those where everything went wrong for whatever reason. But then there is the large bulge (bell) in the middle where the majority are.