I tried them in a few Alain Ducasse restaurants.

Indeed, very nice personalised welcome, usually the very best table plus the restaurant normally serves its signature cocktail upon arrival. I am not a cocktail afficianado, however I was never asked whether my preference was champagne.

Amuse bouche and an additional special dessert are usually offered with compliments. Usually when you receive an additional dessert you do not receive a farewell gift - this depends on the restaurant.

The only benefit I do not enjoy that much is the visit to the kitchen. Maître d'hôtel normally invites you to see the chef ("Who would love to show you around personally and introduce to the team") at the end of the dinner and in a way that it would sound rude to decline. Therefore you go to the kitchen and it feels equally awkward both, you and the chef. I am not into cooking so I try to express some interest and compliment the food, and the chef tries to express some enthusiasm about seeing me in the kitchen.

Apart from that this is a very much appreciated Centurion Card benefit.