I have driven a few in the 300k miles range, one 400k miles, I have taken a couple apart for clogged intakes/cylinder heads and no visual wear in the cylinders at those miles...they seem to last a VERY long time, however some people have bad luck and turbos die, timing belts fail, etc.
Heads always need rebuilding by 300-400k miles (guides, seals etc) but pistons and rings okay still. Way more durable then older VW diesels, I would say very similar to Benz diesel in quality now. Maintenace is key and a careful owner for long life. They are turbo charged...