My favorite store in Paris is the one in St Germain des pres, just in front of the church. The area is also great to visit with a lot of restaurants and cafés.
The SAs are nice and they take the time to discuss with you, they seem to have less pressure than in the very crowded stores (Galeries Lafayettes or Printemps Haussmann).
The 3 storey store on the Champs Élysées is definitely worth a visit though, it's eye candy everywhere. I go there when I need to authenticate an item but it's further from my house and it's much more crowded.