Funny, I was just looking at these kinds of things two days ago. My question is: are there any that you can replace the battery on?
Most of these chargers are powered by standard 18650 batteries. I've also got some flashlights which also use 18650 batteries, so I actually think something like this would be more versatile than having a spare phone battery- it could be used for charging my Android phone, my wife's iPhone, or a tablet.
My 18650 battery charger also has a USB out, but it's bulky since it's a dual battery charger, even though it only uses one battery slot for USB out. And it actually requires more than 5v in for battery charing, so it can't be charged via USB. No, don't go there with using a USB charger to charge another USB charger...
Something self-contained like this would be more useful. I'd also like 2A output, but most chargers would then have to be double battery (discharging a single battery at more than 2A is not that great for it), which brings up the whole problem of needing matched pairs of batteries since they're usually just wired in parallel. Parallel discharge circuits would be needed- I wonder if you could just use two separate chargers and have a Y-cable to combine them into 2A?
sigh That's the problem with getting really into something- your requirements/preferences narrow down so much that it's impossible to find the exact thing you want.