I bought a 2006 X5 4.8is with 50k miles (80k km) about 1.5 years ago. So far I had no problems.

Bear in mind that you need a good repair shop to maintain your engine. The 4.8is has the N62 (not M62) engine. The N62 is a very advanced engine with a lot of specifics, that (in this version) was only used for the 4.8is and one of the 7 series of that time. In my case, I had some engine issues, and I had trouble finding the right workshop with enough experience to troubleshoot my engine.

Also, note that most of the 4.8's come full optioned => which means in turn that there is more that could potentially break and needs to be fixed :-)

Otherwise, the 4.8is is a gas guzzler, but still a solid choice.
Hope this helps.

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BMW 2006 X5 4.8is