I have both Stihl ES and GB Pro-Top & Ti bars in 16-24 inch 3/8 size. I like the GB bars better, and they bend and flex and hold up really well. They also have less of an oval shape than the ES bars. The Stihls are .063 so they may be wearing faster becasue the rails are thinner. The GBs are all .050.
Anyway, for the money, the GB bars are good. I have pinched them in big trees, bent them and bent them back, and no cracks, or permanent damage. Hell, you cannot even tell where the bars were bent. They have held up well. I have cut a lot of firs, redwoods, oaks and maples with them.