go there on business quite a lot - there is a huge FNAC with wonderful cds and dvds at much cheaper prices than the UK (and lots of groovy french and north african music if you are interested in that sort of thing). Apart from that I never buy much. As beety says it is the same as knightsbridge.
Mind you there is one brilliant childrens clothes shop with cool, quirky, casual clothes at prices about 25% lower than Gap called "Au Pareil du Meme"... I always stock up for the children there. It is a chain, but brilliant quality and lovely clothes. Send him off there! And Zara is a bit cheaper there than here if you trust him to buy nice stuff... huge one on the main drag as well.
What else do I buy there... oh yes, diptyque candles are about 15% cheaper. That's it, I think.