I think(aka MHO) all of Mercedes diesels are great but I’m only familiar with working on the 615, 616, and 647. Excepting the latter(opinion) the early diesels are fun for the beginner enthusiast and professional alike. What makes them great to me:
They’re relatively simple, easy to maintain, fun to work on, parts availability, enthusiast support like this forum, and the fact that despite horrendous abuse/neglect there are so many of them still in service. That makes them great to me.
Can they compete with a modern diesel? In some aspects the answer must be ‘Absolutely’! But to be fair a comparison between an OM617 and a Cat 3208, 71 series jimmy, B-series, et al… that’s just apples and oranges. Diesel technology has far surpassed performance capabilities but operating and maintenance costs…? Yeah those have far surpassed the older OMs on at least the same gradient with performance. I think a great diesel engine is only as good as it’s application. There’s a lot more 616 and 617 transplants than non OEM engines transplanted into Mercedes. I can’t recall hearing of one but i’m sure there out there. The 616 is still produced on license to a company in India for new production cars. Considering the number of choices available I think that speaks volumes. They’re still a cost efficient power plant if you don’t need to pull a house off it’s foundation or break a record a Nuremberg. I expect 616 amd 617 will be around for decades to come. I hate to use the metaphor but these engines(in my experience) are like the unconditional love a dog shows his abusive owner. They live better longer if you treat them like you ought to. Abuse them regular and eventually that dog won’t hunt.