I have a soft spot for stained glass, and this little chapel is a supremely fine repository of that art form. Architecturally it is a wonder: the spindle-thin columns of stone supporting the weight of the roof with over an acre of glass in-filling the spaces between, towering to a great height above the decorated floor. And the light. All that glorious coloured light streaming in from the outside. It is no wonder that the architects of Gothic cathedrals thought they were transmitting the immutable presence of God to the awestruck congregations of the middle ages. It still works in our high-tech information age. A telephoto camera lens helps to make out the detailed figures within the higher glass panels that are otherwise indistinguishable from the floor.
I'd love to have had the chance to experience a choral concert in the upper chapel - I suspect the acoustics are impeccable.
The other reason I loved the Sainte-Chapelle is more prosaic: I had my first actual conversation in French with a security guard, asking for and clarifying directions to a toilet other than the nearest one (inexplicably closed...) for Bendy. A great sense of accomplishment swept over me despite the ordinariness of the subject.
Our travel tip for the Sainte-Chapelle is this: there are TWO queues into the Palais de Justice. The one on the left is for visitors to the Palais de Justice (i.e. probably not you). The much longer line on the left is for the Sainte-Chapelle. We joined the wrong line at first, and then were sent on to the end of the other one right at the doorway. It stretched back to the corner of the building, so we thought we'd be there for ages, but it was really only about 20 minutes from there until we went through the security screening checks.