What it is?
The wii balance board is a large platform that plugs into your Wii console and you stand on. The board can weigh you, you can play dance games or exercise on it and improve your balance and posture.

Cost
I have seen the board in black and white. Online they retail at around £69 new. The prices seem to vary quite a lot from place to place and they are often on offer so it is well worth shopping around for. There are also many cheaper versions around which are not official Nintendo boards but I can't comment on how good they are in comparison with the official board.
Weight and dimensions
I find the box bulky and the board rather heavy at 3.5kg without the box, so you wouldn't want to buy it at the start of a shopping trip and carry the big box around all day.

Th board is just over 20 x 12 inches. It is just over 2 inches deep and just fits under my television cabinet which is great for storage when I'm not using it.
How To Use
The board plugs into your Nintendo Wii console using a USB cable. I like the fact that it is powered from batteries as you only need your Wii console and television plugged into an electrical socket and I find many other console accessories come with so many cables they can create a mess. It takes 4 AA batteries or you can use a rechargeable battery pack. The batteries are supposed to last for 60 hours which I think is good as I don't use my board for more than around an hour a day so they last a couple of months.

There is a large power button which has a blue light that comes on when the power is on. It's very simple to set up by plugging in and turning on the power, any more set up required is for each game and you will get instructions on the screen on when to step on or off the game.
Games
There are many games available which can be used with the balance board.
Wii fit comes with the board in most packages and includes both games and exercises to keep you fit. For a more exercise than games based workout another game you can buy is EA Sports Active.

Sports games available for the board include skiing, snowboarding, skate boarding and many more.
There are also many dance games available including Zumba dance workouts and Just Dance.
My Opinion
I think that anybody who has or is considering buying a Wii console should definitely opt for a package with a balance board. They can be bought for a good price, especially if bought used and many games shops sell used consoles which still come with some kind of guarantee.

I think the board adds a lot to the overall experience of the Wii, is good fun for all ages and can be used for both fitness and fun. I know a few people who bought a board and never took it out of the box. I'm sure if they did they would be using it regularly as I personally do as I find the board games more fun than the normal console games.
I was amazed by the sensitivity of it and when you are on the board you actually feel like you are doing the sport or game on the screen. Even though the board itself does not move, you get the feeling that you are moving and controlling the board just be shifting your weight on it. I find it very accurate and realistic.

If children are using the board you might want to keep an eye on them at first as it's easy to lose your balance on it. Also, you are not supposed to jump on the board and it can be very tempting to jump especially when playing games such as ski jumping when all is required in bending and straightening your legs rather than actually jumping.
I love my board and use it often!