I took one out on my last GS, it was around £200 and proved to be a waste of time.
My bike ate a valve just after it went out of the 12-month warranty period.
It was fixed, but my dealer said it was the sort of fault that BMW would have sorted out without charge.
Then I tried to claim the cost of my daily train fare into London while the bike was being repaired - as the warranty agreement covers alternative transport costs.
BMW insisted I couldn't have it as their prior approval was needed before claiming.
I asked how it was possible to anticipate the bike breaking down in order to fulfil this condition. They simply repeated their answer.
Result, two £60 weekly season paid for tickets out of my pocket.