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There's not really much i can say about this, it's an extended battery, 2000mah compared to the 1600mah (i think) that you get with the phone, though if your phone was already nearing 2 years old, it had likely deteriorated a bit, so the jump in battery life will probably be quite significant.
I used my phone with the standard battery quite a bit while at work, and if taken off charge in the morning when i left, it would usually last until i got home, sometimes until i went to bed, though if i needed to go out in the evening, i would give it a quick hour charge before going out again, with the extended battery, it lasts 24hours easily, if left on standby for long periods, 48 hours is possible.
The only drawback is that it's slightly thicker in size, which (with the included back panel replacement) will make the phone a couple of mm thicker, it now lies perfectly flat with the speaker at the bottom, which was protruding before. I actually prefer the feel of the phone with the thicker back, but some might not like it. It does protect the camera lens as well when the phone is on a flat surface.