I love my Impact Gels. And I trail ride for up to 5/6 hours at a time. But I think I love them for their quality felt more than the gel. The felt is really, really nice.
I don't think the gel travels around like other gels might. I think it is more like the gel insole in a shoe. And the felt makes up for any possible issues with the gel anyway. Did I mention I like their felt?
I own a 5-Star as well, and I love it because it is quality felt and light-weight. The Impact Gels I have are older and a little heavier. But they are good pads and even though I own a 5-Star I frequently use my Impact Gels as well.
So no problems to report here. I really love them. And as they are a little bit cheaper than the 5-Stars, I could really foresee myself buying another Impact Gel in the future. Not than any of my old ones have worn out. I bought them second hand from a friend and they are still going strong. They are probably at least 5 years old now.