I keep forgetting to do a write up on a modification we did to the C-Max in December. We went up to the in-laws for Christmas, and although we had planned to take a bigger vehicle to hold everything, once the time got close I was reluctant to burn all the extra gasoline - so I bought a hitch, mounted it, bought a hitch mount cargo box, wired in the trailer lights, and off we went. I did this all in the evening before we left, so I didn't take very many photos, sorry.
The box works perfectly, it will hold our two largest suitcases no problem, and left the back of the c-max for the small suitcases, and assorted pies and goodies that needed special care. All the seats were occupied. The box weighs about 60 pounds i think, they claim a 150 lb capacity, but I will be restricting myself to 100 pounds max in the box, because the box itself is fairly heavy and I am uncomfortable with hanging much more than that on an 1.25" hitch.
The box kept the luggage dry, started on the coast with a rainstorm, ended in the interior in a blizzard, so we were pretty happy. It got totally covered in salt and slush and grime, due to its location, but the inside was dry and clean.
Big advantages of the cargo box are : It fits on the back of the car with the garage door shut and the car in the garage, whereas a roof box would need to be mounted and loaded outside the garage in my case, for clearance issues. It is quite boxy and handles suitcases fine, whereas most roof boxes are too shallow for suitcases. My wife can load and unload the cargo box without getting a stool or stepladder. It doesn't seem to change the fuel economy at all, although I am sure it must increase drag to some extent, hanging out there in the slipstream like that. It's an extra bumper if someone taps us in the snow, or if I back into something.
Big disadvantages are: It is not as attractive as a nice streamlines roof box, in my opinion. It is cute in its own way, but not handsome, if that makes any sense. It can not hold skis or snowboards, although I have since mounted a snowboard holder on the top of the box, I'll have to post that in a separate thread. It blocks the license plate, if you want to remain legal you have to move the plate to the mount that is on the back of the box. This only takes a few minutes, but is still one more job to do. It has tail lights, and they should probably be wired in, even though the cmax lights are high enough that the box doesn't block them. I chose to wire in a 4-pin trailer connection and plug the box into it, and it looks really sharp at night. It blocks the reverse camera, and you will need to cancel the reverse parking sensors every time you back up once it is mounted. I need to be careful how much weight goes in the box, because it will lighten the front end considerably, and change the steering, braking, and traction dynamics. (we haven't noticed any change at all with the 100 lbs in it. I suspect the C-max front end is pretty heavy.)
All that considered, I decided it was the best option for us, and am very happy with it. I will try to link to some photos.