Zanotta Kilt sofa - Comfortable or not?  

a couple of things
I haven't sat it in, but I've tested out a billion other sofas and I kinda know at this point just by looking what to avoid.
This is a deep sofa. If you like to sprawl, it could work. But if you like to sit upright, you will need a couple of pillows at your back so that you can sit forward enough that your legs don't stick out straight like a 4-year-old's.
Loose back pillows often become rather sloppy-looking over time. Arranging random loose back cushions like this sofa has, plus the necessary throw pillows, to look good while still providing comfort and support might entail compromise. I mean, optimal support and comfort placement might not be the most pleasing aesthetically.
One last thing---it sits directly on the floor with no room to tuck your feet under. This matters when getting up from the sofa---it's easier to do if your heels extend back a little. And it's more comfortable to lean over the sofa when your toes can be under it. I hate furniture with no toe-kick!

exactly, all those exposed...
exactly, all those exposed wood sofas weren't just like that for fashions sake, the lower edges of that Zanotta sofa will allways get knocked about, worn and cat scratched before anything else. Much easier to polish up a bit of teak or rosewood than try and fix upholstery.

Actually, the sofa does have...
Actually, the sofa does have legs. The sofa is just very low, so you can't see them, but I still think there would be space for the heels, while getting up from the soffa.
I'm mostly concerned with the low back support. I don't mind adding loose pillows, I think it would look pretty cool with some different Marimekko or Girard fabrics. I'm just afraid that if the back support is too low and hard, it might be felt though the pillows...

ah, yes, I see...
...the legs in the room views. Bad photoshopping on the main pic! They took the shadows out along with the background.
I would call and ask how thick the padding is on the back. They should be able to tell you. It has a very clean, sharp profile but that doesn't necessarily mean that it's hard. A block of soft foam would look the same when new. If the padding is more than 1.5" thick, it will probably be comfortable enough to lean upon.

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Three photos with the legs missing ? Do you suppose they offer it with legs and without ?

I agree, very bad photoshop...
I agree, very bad photoshop work...
I think it is offered both with legs and with some kind of low base. But in any way, the first pictures looks like it is a flying object rather than a sofa...
The used one I have seen pictures of fortunately has legs.