For blind spots, the OEM (but expensive) way is to buy the European aspheric mirror glass. The whole mirror is convex, and the outer portion even more so. Given how big the X5/X6 mirrors are, it's not cheap especially if auto-dimming, but sometimes one can snag a deal on eBay.

OE BMW X5 E70 X6 E71 Mirror Glass Spiegelglas Spiegel bei eBay.de: Karosserieteile (endet 16.10.10 20:15:47 MESZ)

I've been driving with them for years on my X3 and the 325i, and gotten used to their wide range of view (eliminates the blind spot pretty much). No need to turn my head unless I want to make extra-sure in heavy traffic. When I swap into any other car, the difference is immediately obvious. I literally feel kind of blind when I drive with flat mirrors.