Instead of buying extended warranty, save the money for the repairs your self. Believe me, no workshop like to deal with extended warranty companies. Plus, these extended warranties company are very eager to take your money, but after that you get a rude claim adjuster and a lousy 1-800 number with a message machine. Also, if you still decide to go xtended warranty, read the fine print. They only cover tranny and engine. Not the extra's. Bmw I6 are solid build with solid transmissions. The stuff which will go wrong won't be covered by extended warranty. Such as electrical stuff, sensors, radios, seats and other minor nusiance.
Just keep getting the tuning in time, replace fluid's once a year and you would be one happpppppy camper.
PS: thats my 2 cent.