I have a couple of different sized 5-in-1s (one big, one smallish), and I've used them in pretty much every way described in this thread. They're great!
The gold reflective surface is about the only option that I have rarely used (I have tried and rejected it numerous times - one of the few photos where I used it is below).
They're useful for backgrounds (if you have a large one - fnar fnar!), both white and black.
They're perfect for wafting smoke, haze or plain air around, or for directing it where you want it.
They're excellent as reflectors, either for daylight or "studio" light, as a nice neutral fill.
And they're also invaluable as diffusers, as mentioned in the OP.
I've even used the main diffusing 'frame' and the reflective cover separately at the same time (see the second photo below).
I used to use mine as diffusers with a hotshoe flash a lot.
They need to be appropriately positioned, obviously, but they can be a better and more controllable option than a shoot-through umbrella in some circumstances.
Three examples of using a 5-in-1 as a diffuser with a hotshoe flash (in these instances an old-school Vivitar 283) - full lighting info on click-through, if you want it:
Smile! by
Nionyn_, on Flickr
Peepshow by
Nionyn_, on Flickr
Echo by
Nionyn_, on Flickr