The only down sides I can think of regarding the 550 as a DD are finding a good spot to park it and the high visibility of such a car in any given situation...sometimes you just want to slip in under the radar...even with the low key 550, this is still a problem.
My experience with the 550 indicates that mechanically, the more you drive it, the better it performs/more relaible it becomes. I believe this is the case with most Fcars, but even more so with the newer ones, the 550 especially so. Ferrari designed this car as the "practical Ferrari"...so if any Fcar can be used in this fashion, the 550 would be it

As to specifics, in heavy traffic, the clutch can get a little weary, but nothing like my old 355, and Im assuming the BB is a lot heavier.Other than that, traffic is just boring, but not a concern. Overheating is never an issue, even on 100+ days in rush hour stop and go traffic...
Running costs go up with mileage, but that applies to any Fcar, However, gas could get a little funky if these hurricanes keep shutting down refineries.
Again, I cant see this being much worse than the BB.
Hope this helps