I had an excellent near-three week stay at Claire's loft in the center of Paris. In terms of location, one could not dream better: this is right in the trendy Marais neighborhood, a couple foot steps from the centre Pompidou, within minutes of Les Halles, etc.
The studio is very nice. It is on the top floor of a building on a busy street. The building itself is very quiet, but noise from the street makes it to the apartment (though closing the windows is enough to block it off). The studio is very nicely decorated, with a cozy bed on the mezzanine, modern bathroom, small but functional kitchen. It is truly a very pleasant place to stay at, right in the center of Paris. (The only downside, if I had to name one, is that there is no view - two windows let enough daylight in, but there is not much to be seen.)
I highly recommend Claire's place for the central location and cozy apartment. (In addition, communication with Claire was a breeze throughout, she is very responsive.)