I haven't used the flats yet but I have a set of cobalt rounds on my stingray and in my opinion they really do give a far greater sound compared to nickel or steel. Sharp and punchy where needed and tons of depth, especially on the low b. They feel more smooth compared to my old strings, can't really say on lifespan just yet though. Had them on since Jan and they sound just as good now. I used to use elixirs but I'll stick with these from now on, on this bass at least