Just looking at them and wondering if they would handle all off road duties just XC pootling, as well as shopping etc.

They're fine for off road. People have done long distance touring including tracks and broken roads, coped absolutely fine

I've always wanted one but don't have the space to store one (or any other bike in fact)

Unsurprisingly I had one for a few years, rode it every where including john O'groats to lands end. Fantastic bikes, the handling makes the length disappear. Off road they work very well, very comfy due to the length wise flex. The only thing is the length makes it next to impossible to loft the front wheel and can hamper rear traction. I did hundreds of miles off road including racing a cx bike down the ridgeway loaded was Interesting

Highly recommended bike

Off road big dummy action here http://www.ridingthespine.com/.

Oh they also now do the Big Fat Dummy it takes fat bike wheels or you can build it with 29+ wheels

I want one but I don't know why

http://surlybikes.com/bikes/big_fat_dummy

I've got one. Really like it. It's mainly for carrying the little ones (will fit both on the back comfortably) and shopping.
Although looking forward to touring a bit when the little ones are a little bigger.
Rode it round the Bristol singletrack with slicks when I test rode one, and seemed very capable (although better off road tyres would help)

Did a mixed road/woods ride at the weekend. You do need to think three turns ahead when snaking through the trees. When you're moving it's fine, slow manoeuvres are a bit fiddly. And just picking up the back to turn 180 is a non-starter.....

I want one but I don't know why

This is where I am at the mo.