Not great.  maybe just mine.  The box is warped/bowed. The center is higher than the edges.  This is not something you could cause with a bad install, and you cant fix it with a good one.  

Also when you have it flat on the wall it slopes down towards the front.  Put something on this and it is likely to roll or slip off.  You can add shims to the bottom between the mantle and the wall and then drill  holes at a 45 at stud locations  through the top and screw it down to the wall.  That should do the trick to keep it flat, aka parallel to the floor. 

Overall for the modifications required and the warped box which cannot be fixed I would look elsewhere or be prepared to do what I am having to do to make it look right.