Being a thin person I have always encountered problems with purchasing the armband that fits on my upperarm without falling off. After reading the other reviews of this product I decided to give it a try, and the product is an amazing fit for me. Also lent it to a female running partner and she told me it fits perfectly, too!
The chamber for the phone is crafted specifically for the dimensions of iPhone 4/4S and iPod Touch 4G, and you will have to take your bumper case off if you want to slide your phone in. However, it is not a problem for me and I don't worry about the phone falling out, because it fits the phone snugly (and sometimes I even have problems getting it out, especially with sweaty hands).
The plastic screen is conductive and therefore you can still use your phone when it's in the armband, but at the price of reduced sensitivity — but it is still good for thumbing through your playlist, checking out your workout pace and etc. So far I have used it for a few 10–12km run and 30–40km bike rides, and NEVER had a problem with it.
The fabric is not waterproof though, so I would not recommend using it in wet weather. If you sweat an extraordinary amount then you should also be wary that your sweat might soak into the phone holder, as it is not waterproof.
You should also be aware that if you are a normal person who works out, or have a slightly thicker upper arm, you might want to explore other options. For a skinny person like me, it fits very well but I can see it being too tight for others.