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I bought this as I really love the boxing game on Wii Sports (I turned pro on it in about two days, waking up with surprisingly sore biceps!!). The drawback I found with the standard Wii nunchuk is that the cable length is not long quite enough for an average-sized adult who, ahem, might get a bit too carried away with all the excitement of a new Wii.
How it works / installation
The nunchuk is freestanding and contains a rechargeable battery, which is very easy to charge as you simply plug the nunchuk into the mini USB cable which comes with it. There is an LED light to indicate battery charge. There is also another part which attaches to the Wii controller, in the same way as a Wii Motion Plus unit does. Once both are connected, there is a button to sync the two, and then away you go.
It does exactly what you would hope from a normal nunchuk, but with the advantage of having no wire to accidentally wrench out of the controller. As I hoped, the nunchuk is as responsive as a standard Wii nunchuk, so in total, you gain the wireless advantage without much of a downside. The battery also lasts a good while and I've never had a case where it has lost its juice while playing. If this were to happen, then I'm sure you could plug in a corded nunchuk to tide you over.
For me, it certainly made gameplay more fun: boxing felt unrestricted and I could really let rip. Also, when playing Resident Evil 4, I could really get immersed in the gameplay, which at times makes you jump out of your skin and want to run for the hills!
I was going to entitle this section downsides, but they are not that bad. If you are thinking of buying this product, be aware that the controller attachment makes the controller longer, so if you have a silicone casing, you will need to take it off, and therefore lose its ergonomic effect.
Finally, the John Lewis price is far too high: I picked my mine up for at most 8 quid (I think it was more like 4, but can't be sure) in a HMV sale. It was exactly the same product as John Lewis's, down to the packaging colours. There should be better prices if you shop around.