I got a couple of these soft flasks at the same time I got the 6L Duro last summer. They make it a very flexible and adaptable system. On a shorter day, one or two of these is super convenient. If it is long or very hot day, I can use them with the bladder and not worry about the late day mystery of whether or not I am about to run out of water. I haven't had the need yet, but I could also see using one in a side pouch of my backpack for the same reason. They have worked without a hitch, and I find the short tube and bite valve super convenient when trail running. I know a lot of runners who would frown at those extra grams, but for me they are luxury worth the minimal additional weight. The screw tops are designed to ride comfortably in the front pouch of a vest. The opening is too small to use with ice, but that has not been an issue for me. If it long or hot enough for me to be thinking about ice, I have just used it in the badder and all is good. Overall, I've had no problems and am very happy.
