What I truly find disturbing is that the GM companies have managed the debate to include only the issues mentioned here in this discussion: costs, viability, litigation, effectiveness and humor (as in laughing at the "anti-scientific fools who don't trust science"). I am replying to this post because the P fell into the trap: "the last twenty years."
Well, let grandpappy tell you about the bad old days, when "science" was used to support the growth of an industry that was producing a great product, making it cheaper, tastier and more available to people all over the world who wanted and needed it. People who were advocating government controls, advertising against this "dangerous and harmful" product ("it can cause cancer!") were being laughed at and called scaremongers and health-nuts and all the varieties of names used against the anti-GMO crowd today. The product, like the new GMO products had been around in many forms since the dawn of time and had been used safely by humans for a large part of it.
But, in the 50s and 60s, crop scientists started to mess with it. They also worked on its delivery and how to make it appealing to more people. They advertised to a wider market and made it more .. palatable? ... to more people. It was a wild success and spread around the world faster than a wildfire. Concerns about health risks were knocked down as not being empirical, not being valid and not being "scientific." Marketing was rampant, TV, radio, billboards, everywhere people saw it and bought it. I remember in the 80s, seeing an billboard ad that caught my eye for one of the variety of forms of this product, it involved a subliminal of a nude woman spread-eagled across a small tin of the product. That billboard stayed up for months, nobody said a word, even there within 5 miles of Jerry Falwell's Thomas Road Baptist Church!
If you haven't guessed, the product was tobacco. When I was 7 years old I started smoking, and continued for most of the next 42 years. The science that was argued was the difference between causation and correlation in the incidence of cancer. The original data about correlation was done in the 50s, but it was done by the tobacco companies themselves and they buried it. It was a whistleblower who brought the data out, and he was ruined because of it. It was 30 years or more before the government banned cigarette advertising, but other forms still were advertised into the 90s AFAIK. Scientists could prove that there was no data to support causation, therefore cigarettes were not cancer causing. Correlation data took generations to amass, so by the time we had it we had hundreds of thousands of deaths and mouth and tongue and throat cancers, lung transplants and emphysema, COPA, all from tobacco.
For these reasons I don't feel the need to point to science and say "there is no proof." I do feel the need to be mistrustful and cautious when a mega corp says, "Our scientists can prove it is safe!" I have heard that before and believed it then. But you know the old saw "Fool me once shame on you, etc." I won't be fooled again, no matter what the science that the company produced says. It will be generations before we know what the downside (and there is always a downside to everything that happens) is and how bad it will be.