I give him a B as in "builder". He's a consenus builder. After Donohoe's reign someone had to bring the franchise together, get the coaches input on players and scheme, and make Ralph happy. Marv is not really the GM, he's like a "chairman emeritus" or "chairman of the board".
I think he's done his job well. The coaching is better, the draft was not a total disaster as was advertised, team play was better, etc. Bottom line: at least we didn't go 14-2 and then let the staff who took us there walk away!
Year #2 will tell us more about how Marv is doing his job.