I've written, favorably, about other cases from UNU/iBattz before, but their latest case, the UNU DX Plus, packs even more power. With an extra 2400mAh of juice, you can more than double the battery life of your iPhone 4 or 4S.
The case does add noticeable bulk thanks to that large battery, but they've made a nice design difference from the other case I reviewed earlier. Specifically, they use a slider style band so it's easier to remove the iPhone from the case.
If you have a long flight or work far from a power outlet, such a case is a nice option. I've been able to go all day with their older model, and my battery still read 100% full, so this larger capacity unit promises to offer even more battery life, perhaps all weekend! This will be very useful during trade shows like CES or the upcoming WWDC where even if you can find an outlet, it's probably taken by some other power hungry gadget.
Apple has moved away from replaceable batteries in their laptop line, so it's unlikely they would ever offer that in an iPhone. It's nice to see an option for those of us who need more power than the stock iPhone can offer.