It does not supply 24 hours of battery usage. That is only a marketing tactic. I was given the battery to test it before it was released to the public. At most, it will charge your phone from 0 to about 80-85%. The build is a little shoddy and not something expected of a fifty dollar item. Also, the dock that enters your iphone is built asymmetrically and I was unable to plug it in with a case on my iphone.
Well yes, Duracell is a trusted brand. (I prefer Duracell batteries over any other brand) But as far as an external charger for mobile devices, there are just so many better devices to use than what I think this guy is talking about. While yes, this does look to be decent.. it is only 1850mAh. (source) This will successfully charge your iPhone...barely. Even though your iPhones battery is only 1450mAh (i think) it will take more battery to fully recharge it. Honestly, I would not even recommend a battery less than 2000mAh.
With that being said, I have recently worked beside the company NewTrent with many of their products. I have tested four batteries by them. (sadly I do not have reviews up yet) The iDual, iTorch, Trident, and iCurve. (I'm not addicted I promise ) But these batteries will require you to bring your cord along with you. but this makes it so you dont have to have a weird case or anything like that. It also gives you the ability to charge any other device you may have (and some have two ports to charge multiple devices)
Finally, I will not only endorse NewTrent despite they are my favorite due to their diverse selection and flawless customer service. The only other company with comparable batteries would be Mophie and they are a little over priced. You want the best bang for your buck, go grab a iCurve (7000 mAh) for ~$45 and charge your phone 5 or 6 times before having to recharge the iCurve itself. but if you got anymore questions, let me know. I love talking about this stuff. Hope this helped!