Bloody brilliant car.
I had one and it was one of the best cars I have ever owned, I had it from new though. I have recently started to lok around for another and from the research I've done there seems to be a few things to look out for.
Chassis, yes they rot. They rot from the boot forwards so the rear seats and floor pan rot first. Then the front chassis members where the engine is bolted to. The engines are pretty good if well looked after. Not a lot to go wrong and most change the old points to electronic ignition (easy to do) The wheel bearings go on the front as theres a lot of load on them. Brakes are pretty much fine. Interiors the seats get worn on the drivers and passengers easy, the springs in the pram type seat rails stretch and the covers rip. The hoods are/can be leaky so make sure the seal around them is ok, again an easy fix.
Parts are pretty good to get hold of, 2CV Owners Club a great source of info/parts/breaking cars.
Of the 3 I've recently looked after, 2 were in excellent condition and drove really well and had the chassis replaced they were a bit more than I wanted to pay (for a third car) so I didn't buy either. The 3rd car needed some work, it has an MOT but the roof needs replacing (can be expensive) and the bars that hold the lights on were really rusty and so too the grill area. I'm still negotiating a price so we'll see.
As far a space goes, well I used to get 3 windsurf boards, 5 sails and me and my mate in it no problem. If you take the rear seat out and open the boot up you have masses of space, I used to open the roof all the time.
They are not the speediest of cars but easily do 70+mph and pull really well, its not a sports car so don't expect quick excelleration, mpg wise I had aves of 50+.
I'd have another.