I guess I was a little disappointed, so I wouldn't want to get anyone else's hopes up.
Imagine half a dozen underground malls that are connected. That is pretty much it. If you enjoy food courts, and shopping all day without seeing daylight, then this might be your place. But the shopping above is better, in my humble opinion. And the food as well.
I don't find the underground as charming as I find it convenient, but if you like spending your day as a mole, then I think you'd like it here. I have lived in Montreal for 6 months, and don't use if very often.
And yes, it is clean, and it is impressive how big it really is. It is huge.