I have had two very different experiences at Yauatcha. The first was a Sunday evening eating upstairs and was wonderful. A great waitress who recommended wonderful food from the menu.
The second was terrible, a large group (which they really aren't used to) for my birthday. I was stressed for orders and presented with the bill as I was about to blow out the candle on my cake. Hmmm.
If you go, stay upstairs and don't go en masse.