Despite some reviews saying that this armband didn't fit properly, I did some careful measuring before I ordered (since not all people do) and determined it should fit my needs. Turns out the other reviewers were right.
I measured my arm and it was within than the specs listed on the manufacturers website that it should fit 12-17". It was still about an inch and a half too large. I was able to work it out by using my sewing machine to shorten the strap, but I was not very happy about it.
My phone is not an iphone, but it is about the same size. When I put it in the pouch, the stitching holding the pouch to the armband started to rip. Once in the case, I could still use the touchscreen, but it was not easy in the least. I basically have to take it out of the pouch if I want to do something other than look at the phone.
Overall, it's cheap and usable, but those are about the only benefits. It isn't very good quality and doesn't actually do the majority of what it claims.