I paid about £250 for 3 year's of tyre insurance when I bought my Z4MC, and needed one about 2 weeks later due to a pothole slashing the side. Cost of 19" PS2 was £290 from the dealer, and they sorted it all out for me - including recovering me and the vehicle to a local (to my destination 150 miles away) dealer so they could fit the tyre.
Never had the need to use it again though, and it's now expired.
It wasn't a BMW policy, but something Specialist Cars were selling as an add-on - documentation says Specialist Cars and Auto-Protect MBI Ltd.