I tend to think of black bags as for cooler temps, so it gets used rarely in summer, but it was my main bag last winter (my first year with it). I even traveled to Paris with it (for six weeks), so I know it withstands the wear of close quarters in the city. I also fearlessly carry it in the rain. With help from Docride (in the vintage bag care thread), box (and many other classic leathers) can be worn in rain without worry.
That said, I'm a decided lover of classic H leathers. Most of mine are box, chamonix (a matte sort of box), vache naturelle, barenia, and toile. I love patina! Plus, I find Clemence or Togo to be heavy. My one Clemence bag (a Marwari GM) is a beat to carry for very long. The 35cm Kelly is a big, heavy bag. Box keeps it lighter than it would be in Clemence or Togo.
Ease of use for a Kelly is ... to individual taste. I do find the Kelly fussy in general, but the retourne style is MUCH easier to work with. If fastened like described before, I can flip open the top flap and slip my hand to grab phone, keys (attention!!), wallet, etc. I don't have a sellier Kelly, but my Dalvy (see avatar) is close in design, and that is far less easy to get in and out of.
For ease of use, I can't praise the Bolide style enough.