The E6x platform is solid, M57 is a brilliant lump but you can't compare it to the newer stuff for economy (but like you say why buy a 3.0 diesel for economy!!)
Mine has tested my patience in the past 120k miles at times, but on the whole been great (E61 has more issues with air suspension, tailgate wiring, etc!)
On 183k now, has been mapped for the past 100k+, and does a LOT of towing. Its still on the original turbo and orignal DPF. I had a DPF lifespan counter expire, and back pressure readings are still low... still there and still spot on. I've done the thermostats and glowplugs a couple of times in my ownership, which has probably helped.
I had the infamous 6HP26 clutch E failure around 130k miles (Box was serviced @ 100k), which was inevitable. Box rebuild and new torque converter.... flawless for the past 50k+. I keep thinking about changing to an F11, but the E6x just work, and are relatively cheap to maintain ?