I've got the Power Pak +, and it's fantastic, my 3DS' battery lasts a lot longer.
The only problem with it is because of it's bulk, you lose a fair deal, like:
-Protective Cases won't fit.
-Peripherals won't fit.
-Can't charge with a cradle. That means you lose the convienience of dropping it an a cradle.
-Extra bulk means you'll have to adapt to the feel of it.