Softbox For Speedlight - Good Idea Or Not?

I am a fan of the softboxes that I own, but I only own two - and they are strip lights with grids. They do rim lighting a treat but I do not ask more of them. I've owned other softboxes before - a big octagon and an absolutely enormous rectangular one. They functioned well, but ultimately were supplanted for my studio purposes by umbrellas and beauty dishes. Personal preference and prejudice, if you will. The use of a softbox with a portable flash is a little more unusual - people who opt for this have a need to take the soft lighting out into the field. They'll have to deal with how to fire the flash and how to support the rig, but these are simple basics - a folding stand and a radio trigger. The benefit will be a far softer and more workable light than available with a bare speedlight. Worth having for portraiture and fashion shooting - nearly essential for wedding work. Is it going to make a difference whether you get a 40 cm or a 50 cm unit? Not really in the image - but you may find the larger one more difficult to carry about. You will be carrying it about set up rather than taken down - complete with light stand - because we all do these things. Your car will become a nightmare until you finally collapse it into its bag. Try not to drive between jobs with it balanced in front of you across the steering wheel.