Well worth £90. It will not be easy to retro fit and it probably costs BMW more not to fit it to all bikes, The left hand bar switch is different.
The functions are fairly accurate and useful. The fuel gauge is only an indicator and you need to set your trip meters to ensure you have enough fuel. The temperature gauge is useful and works fairly well.
The functions are
1) Current fuel consumption.
2) Average fuel consumption (69mpg on my 800, 74 mpg on wifes 650 works out when checked at refueling.
3) Distance to go on current fuel level ( Can be a bit odd at times)
4) Average speed (Tried all sorts but seems to always come back to 33 mph)
5) Ambient air temp,
The real useful function though is the gear indicator.