We obviously have a Sprinter and I know a lot of Sprinter owners and a problem with service has not been an issue with any I know. First of all, not much service but routine service has been needed. Secondly, routine service can be done by just about any garage. I used to go to Dodge dealers for service when they were part of Daimler but realized I didn't need factory service anyway so have never gone to a Mercedes dealer.
With 140K+ miles on ours, it is still going strong. Have driven it to Alaska twice with no fears.
Judge Sprinter maintenance by owner comments, not speculators. Sure, there are probably some bad experiences out there but I'm pretty sure they are no worse than any brand and better than most. I would make the same choice again with no 2nd thoughts.