I have it and it is pretty good. The material is thin and light - and you have to snuggly slip the iPhone into it. It probably will tear at some point. Until then, it is very good. My local Apple store carried another arm band but it was much larger and clunkier.
I bought the Belkin FastFit Sport Arm Band from an Apple store for use while running with my Ipod Touch 32GB 4G. On the 3rd run the Ipod slipped out of the tiny Velcro enclosure barely holding it in or reaching its Velcro counterpart sufficiently enough to hold it in esp. while running! The Ipod smashed on the pavement, the glass broke and the backing got dented. I iitiated a claim with Belkin and they are willing to pay for the repair, however the repair shop says it only warranties the fix for up to 90 days. I have forwarded the communication thread to Belkin regarding the limited warranty to support my new request that the claim result in completely replacing my Ipod so I don't get mine back looking fine but with no idea how the life span has been affected by the internal impact.
Belkin escalations department has also offered me a new case from their website, swearing that they have changed the design and no longer have the disastrous tiny velcro enclosure anymore in the line of Sports Bands.
So, you should be alright...just make sure it fully encloses around the Ipod and there is no chance it slips!!
I bought this arm band and I like the light weight but the band itself digs into my arm on long runs and is very uncomfortable. I have had this issue with several arm bands. Not a bad product but just not for me.