I install roofing and gutter and in my opinion nothing will be 100%. But of all the products out there I think that gutter filter is the best. If hail hits it it bounces off, leaves blow away and bugs cannot enter the system. All the people worried about pine needles, well not much will stop them. they don't blow away even on the roof I have people call to have the valley's of their roofs cleaned of pine needles. nothing is 100% in this world nothing, but of all I have seen gutter filters are the best.
I have to agree with Nicklhead on this one. I have a country home just outside of Montreal, Quebec with 200 feet of eaves, and I tried a couple of different covers and gutter gadgets before I came across the foam gutter filter concept. The gutter filter I chose is called Gutter Stuff and it has more than lived up to its claims. It comes with a 25 year guarantee but I don't see myself needing it!