My 08 X5 4.8 has been very reliable. It now has a bit more than 50k, has extended warranty and maintenance up to 100k. My last X5 4.4i went up to 110k or so with just regular maintenance so I am expecting similar reliability with this 4.8. I usually put the car through pretty decent work out and put about 70-80 miles/day.

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2011 X3 35i
2008 X5 4.8i
2003 330xi -sold-
2001 X5 4.4i -sold-
2001 M5 -sold-
2001 M Roadster (SCed, Highly modified) -sold-
2000 M Roadster -sold-
1995 325is -sold-