How many miles does the vehicle have? There have been only very isolated accounts of complete transmission failure in the 5-speed GM automatic for the earlier X3s. This definitely isn't a widespread failure kind of issue. Automatic gearboxes in BMWs (sourced from ZF or GM) usually have a lifespan of around 100-120k miles (200tkm); some get slightly fewer and others get more.

I drive a lot in mountainous and hilly terrain so I do put more strain on the transmission than most flat-landers, but I'm at 87,5k miles and the transmission shifts fine. Yes, it does downshift 5-4 frequently to maintain speed since I have a 2.5i but it's needed to maintain speed.