The box is very light, less the an the top box on my wife's scooter. I have done a 50 mile run on the motorway with it litterally stuffed to the brim with fruit so certainly over the suggested weight limit (don't ask) and hardly noticed it was even there (apart from filtering in tight traffic at 50 odd while going over cat-eyes - wobbled a bit more on them than usual.

If you're gentle with the lock I've not had any problem with it. Quirky but very livable with.

Its just big enough to carefully get my full helmet in. Overally I'm very pleased with it. I think it looks good on the back and my occasionaly use it works very well.