TeknoCreations claims each battery provides up to twenty-five hours of play, though I’m guessing that’s if you’re only using the Wiimote on its own. I averaged around ten to twelve hours per charge (completely draining a full battery), but that was with a Classic Controller or Nunchuk attached and doing very heavy gameplay. That wouldn’t be much on its own, but since they’re lithium ion batteries you can always ’top off’ the charge by putting it on the charger between games. This doesn’t cause ’battery memory’ problems like it would on Nickel Metal-Hydride rechargeable batteries (mostly obsolete -Ed.), and this is a huge perk in the InCharge’s favor.
While the merely average battery life can be worked around, there is one more notable problem: Wiimotes cannot determine how much energy is left in these batteries. This isn’t entirely TeknoCreations’ fault (though an explanation as to why is beyond the scope of this review), but it nonetheless means that if you don’t ’top off’ your charge, you can and will have the Wiimote suddenly turn off mid-game. But since you can, you might as well so, as to avoid this problem. 
Do note that TeknoCreations also produces additional batteries as a separate product. You do need the original charger to make use of them, so this would presumably be useful only for those with three or four Wiimotes. Still, if that describes you, the extra batteries are about thirteen bucks each; a fair price, I think.
As for the main product itself, $35 is a good price for what they offer. Convenience, reduced Wiimote weight, and long-term savings more than outweigh the minor problems the product has, and it comes highly recommended to anyone tired of swapping batteries.