I bought a '10 M5 with 6K miles on the clock a couple of weeks ago, and also decided to explore extended warranty options as the car can get a bit expensive to service out of warranrty. As far as I can tell, the big 3-4 extended coverage companies are:
* BMW itself (the dealer offered both BMW coverage and their own, dealership-specific coverage)
* EasyCare
* NAC
* Zurich
Upon researching options, it became clear that comparing one policy to another is tricky business. Not only do terms vary (e.g. years and mileage), but the effective date of individual terms vary -e.g. one policy may take effect the day you buy the policy (as it relates to the months/years of coverage), while the miles may have started counting down from the day was first put into service - this is how the BMW plans work.
On top of that, BMW appears to offer a gazzilion permutations - not just varying terms, but varying coverage such as powertrain only, powertrain+electronics (but exl. nav and radio), and eveything.
Anyway, to cut a long story short, both BMW and EasyCare wanted over $7K for 6 year warranty and 100K miles, bumper-to-bumper. NAC offered an "exclusionary" bumper-to-bumper policy for 7 years/100K miles for $4K. "Exclusionary" means that it covers anything not specifically listed in the exclusion clause - which was smaller wear and tear parts. As a side note, I asked them to quote for the same car but assuming it had 40K miles on the clock - that shaves 2 years off the clock, and added $900 to the premium. So it would appear that signing up early actually saves you $...
Moral of the story: it pays to shop around (I'm not done shopping around yet!). Also, I'd worry a lot less about battery coverage, and a helluva lot more about the powertrain and electronics on our cars
Edit: also make sure your local/preferred dealer wants to work with the warranty company in question. Some dealerships apparently refuse work with some, as getting paid proves to be more hassle than it's worth. All of the above companies have good reputations, as far as I can tell, on this basis.