I really like him. And I agree with much of what @SRCJJ has to say, personnel isn't the issue here, as we might have known seasons ago, and yet it's more to do with the system and it starts with management. I think it's fair to say that Wenger could be doing more to get this team working together but without going into another strewn out discussion of Wenger's shortcomings, I think we've got a really good CB in Mustafi.
I, like @Cristo think his ball playing abilities really serve as an important outlet for the team. During his absence, with Kos and Gab, we lacked that outlet to play from the back, to get ourselves forward in many ways and not invite pressure from a team that sniffed out this trait early on. Shkodran often passes the ball through the lines and sometimes into the final third, it makes our lackadaisical build up switch in an instant and can really put the opposition on the back-foot. Combined with his mobile nature, great reading of the game and insta-partnership with Kosh, I'd say we have our most competent CB pairing.
That's not to say it's the most polished either; both of Kostafi are prone to a mistake every now and then, sometimes costly. I think leaking goals is more a culmination of things we could all quite clearly outline in an essay worth of text.