I've used this for a few months and it makes the HTC One much more grippy, protects well against scratches and importantly it fits well. I take it off almost every day to plug in battery packs and it hasn't lost the tight shape. Excellent value.
Works well, build quality is good (not excellent only because the top part which slides up to allow you to insert your phone, isn't too sturdy and a bit vulnerable and it's made of cheap plastic). I highly recommend this because it works well and the cost is low enough for you to buy a couple if you're a heavy user. Holds about one entire charge.