I bought this armband one month ago and after running with it about 10 times I'm quite happy with it.
PROS: Looks great, It stays in place, holds my Iphone 3GS very firmly, it's not uncomfortable, very light, I can control the screen quite easily
CONS: The key pocket only has room for one key. I need two to get into my appartement so that's a bit annoying. The other thing is inherent to most if not all armbands (as far as I've been able to research it online). The GPS signal does weaken considerably because the phone is pressed against the arm. With clear skies it doesn't seem to be much of a problem, but in bad weather it sometimes fails completely.
Still I would recommend this product.