I bought the StuffSafe 80 for our recent motorcycle tour around Europe, as the panniers and top box would be full and we wanted to wander around the places we visited not dressed in, or burdened down by our biker gear; what a brilliant investment it turned out to be!
When it's packed up it's small and light enough enough to slip into the topbox (or in our case strap to the topbox rack), then it expands like the Tardis when you need it. We easily got all of our bike gear (excluding helmets) into it with a bit of room for more; so that was two pairs of boots, two Knox back protectors, my bike trousers and jacket, the babe's dungarees and jacket (all with CE armour) and both pairs of gloves. If you're on your own it would easily take all of your gear including your helmet too :-) Our bike has a facility to attach the helmets, so they didn't need to go in and wouldn't have gone in anyway.
Whilst the StuffSafe wouldn't stop someone with a pair of bolt cutters, it will certainly protect your gear against anyone but a determined thief I think, though the padlock provided could probably do with being a bit meatier I would suggest.
I'm very happy with this product :-)