I've been there. You should know that they have several stores/tea rooms in Paris, though. One is in the Louvre, for instance.
It's a nice place to go with someone to have a cup of tea. I don't know who you're travelling with but for those who don't like hot tea, they also have drinks like iced tea/fruit juice "cocktails" that weren't bad at all.
Obviously, you can also buy all kinds of tea in the store and some teaware.
- Very expensive, in fact it's rather overpriced.
- The teas I had there (1 Japanese, 1 Chinese) were all brewed "western style" rather than traditional. They were still good though.
That being said, it was still a very enjoyable experience. It's just great to enter a store that is so fragrant and all about tea
I would definitely check it out if you have the time.