Nyko sent Joystiq one of the packs, along with the optional Charge Base cradle (the Power Pak + uses the normal AC adapter, but is incompatible with the dock that ships with the 3DS) and I decided to put that claim to the test, and evaluate the device in a non-battery-life capacity as well.
For the last few days, I've been running battery tests, using both the Power Pak + and the stock 3DS battery (as a control). For the sake of consistency, I used the same game throughout, Nintendogs + Cats. I left it running for as long as the system would hold out, with minimal interaction except for occasional play to prevent screen burn-in (if that even happens anymore) and because I'd feel like a psychopath for neglecting my virtual puppy. I did mute the volume in all tests, entirely out of concern for my own sanity.
For both batteries, I tested what I considered the extreme usage conditions. First, I turned the 3D slider all the way up, turned wireless on, and turned screen brightness all the way up. Then, I ran tests with 3D off, wireless off, the screen brightness at its lowest level, and Power-Saving mode on. I ran each test twice and averaged the results to ensure accuracy.
I'll stop with the suspense: does the Power Pak + work? Pretty much! Here are the results of my experiment.
3D on, brightness maximum, wireless on:
Nyko Power Pak +: 5:33
3DS battery: 2:52
3D off, brightness at minimum, Power-Saving on, wireless off:
Nyko Power Pak +: 9:04*
3DS battery: 4:40
*only tested once, because it took nine hours
The good news here is that, yes, Nyko's battery does double the runtime of the 3DS. The bad news is what we've learned about the 3DS's normal battery. In two tests, under conditions many players are likely to use (in fact, the settings under which the 3DS ships), the battery didn't even make it to three hours. That's even lower than Nintendo's already-low estimate. The other bad news that I must mention is that the first Power Pak + sent to me did not work -- it held no charge at all. Nyko quickly dispatched a replacement and informed me that review samples often come from pre-production batches.
Aside from its life-extending functionality, the Power Pak + is a nice product. While it makes the 3DS about 1/5 thicker (which could be a problem if you're pocketing it somehow), it's still comfortable to hold, with a nice grippy texture. It installs in a few minutes, using the included cutest screwdriver ever. And the optional Charge Base comes highly recommended: though it's an extra $10, it restores the drop-in charging convenience, and is in general much nicer than the stock 3DS charger. It has big rubber feet that keep the cradle from sliding on the table, and huge LEDs on either side to indicate charging status.
If you travel with your 3DS and don't mind adding a little junk to your 3DS's trunk, and if you don't mind plugging the AC adapter in to charge (or buying the optional, and really nice, Charge Base), It's not hard to recommend the Power Pak +; in fact, it's mathematically easy to recommend -- it lasts longer than the 3DS battery.