This Armband is a really great product and at the price of £2.95 (30/08/2012) that the Amazon Marketplace seller 'King of Flash' are offering it at is a genuine bargain.
I use mine for cycling, but have also used it for jogging, and can report that it holds the iPhone firmly in place so that you are not constantly worrying about it falling off. and it is also very comfortable, so much so that I often forget I'm even wearing it!
The manufacturing is clearly of a good quality and I'm pretty sure that as long as it is treated appropriately it will last quite some time.
It has a considerable range of adjustment, fastened by a velcro strap, my girlfriend has regularly borrowed mine and it fits her perfectly (and thankfully she has much smaller arms than me!).
The clear protective screen allows you to reach all the necessary buttons, and it is suprisingly sensitive.
The only thing I would say that it lacks is that its not fully waterproof; the strap itself is fine, but the iPhone is not completely sealed within so it will need to be removed if there is a downpour (as I have found out, because of my uncanny ability to summon the Rain Gods whenever I decide to go for a bike ride!).
Highly recommended, you can't really go wrong at this price!