It's rated at 10260mAh, which means it is about twice the capacity of the Panasonic's own 5400 model.  Unlike the 5400, though, the QL301 model does not support wireless charging.  There are probably two reasons for this:  First, it's too large to fit on most charging stands.  Second, it would take a very long time to fully charge.  

So what does 10260 mAh mean in real-world usage?  It means you can leave it in your bag or packpack and forget about it.  With smaller batteries, you have to remember to recharge them when you get home if you've used them that day - otherwise you might end up with a dead phone and no battery.  Even if you keep a spare at home charging all the time, you still have to swap the spare with the used one.  With the QL301 that worry is basically gone.  Even if you use it every day, you can probably charge it once a week.  It can charge even the most power hungry phones with the largest battery sizes 4-5 times.  It can charge other USB chargeable goodies like cameras and WiFi routers, and yes, even tablets like the iPad or Sony S series.  You can charge more than one without worry that you will run out of power.  In fact, it seems almost limitless.

So who needs such a battery?

Anyone who goes out early and stays out late, often running out of power on their phone.  If you are someone who leaves the house early, and ends up at karaoke with your co-workers until 4am, you might need this.

Even for someone who is less of a party animal, having the battery pack in your bag means that you never have to worry about your phone battery draining at an inopportune time.

Perhaps most importantly, this device is for people who travel.  In fact, it's perfect.  Let's say you spend all day on sales call, and just as you get back to your office at the end of the day to pack up and go home you get a call that you are needed in another city first thing in the morning.  You know you have to head out right away, but worry if you will be able to call the hotel when you arrive.  No more worries!  On the other hand, most people who work in the office have a computer with an open USB port or an AC outlet they can use to charge their phone if need-be.  (And many people keep a spare charger at work, just in case).    When you are in the car, on the plane, on the train, etc. all of the time, though, things get much more difficult.  Especially because on the airplane or train is when you often want to use your mobile devices - whether to get work done or relax a little.  If you have a laptop you could use that to charge your phone, but then you are killing your laptop's battery life.

In short, some people really need the QL301, and for others it's just an insurance policy.  I would highly recommend having a rechargeable backup battery - but if you think this one is overkill, just but one of the smaller models instead.  

At the time of this writing, the price of this battery is around 10,000 JPY (~$100 USD).  That seems like lot for a battery, but bear in mind that official replacement phone and camera batteries generally run around 3000-6000 JPY in Japan.  Sometimes you can find them online for 1000-2000 JPY, but sometimes those unofficial batteries die quickly too.  All in all, considering the capacity, I think the price is fair for a product from a first tier manufacturer.

No product is perfect, and I don't expect the QL301 to be either.  That said, at the end of the day, it's just a battery, so there's hard to be much "wrong" with it.  Either you like the price/size/weight/utility trade-off, or you don't.  The main qualm I have about the product is that if you are carrying around a battery that large anyway, you should be able to use it to recharge your laptop too.  (Some units, like Mr. Battery, are designed for this).