We got one so we wouldn't have to take the trailer......anything hooked to your towbar costs you a fortune extra on the cross channel ferry, and heres a handy tip- if you go over the Dartford bridge with a trailer on make sure you go through the autotoll as you are actually supposed to pay extra for them, and they catch you for an extra few quid in the manned booths!!! The other benefit of a box over a trailer is you're not limited to towing speed restrictions on the motorway, an important consideration if youre going a few hundred miles.
A roofbox is dead easy to use- if you get a modern one with quick releases you can get it on and off within seconds, but you're better off to use soft bags in them rather than suitcases to make maximum use of the space.
Because of the loading restiction for your roof (check your manual) they are best for lightweight but bulky stuff- we fill ours in no time with bedding, shoes, footballs, coats, etc- all the lightweight but awkward stuff, and put the heavy stuff in the boot.
Try and spend the extra for a long streamlined one rather than a square boxy one- if you go abroad and you use the autoroutes you could be cruising for hours at 85-90, and the shape can make a lot of difference to wind noise and fuel consumption.