I've been here three times recently (not out of choice – not mine, anyway, after the first time) – once for dinner and twice for lunch. I had great expectations, as I'm sure many people do who visit for the first time.
If you too have great expectations, and think you're in for one of the finest Japanese food experiences in the world/ the UK/ London/ that little bit of Mayfair, then please don't go. If you don't care about the food, and you want either an evening spent with raucous 20-somethings and 60-something hedge-fund guys with their 30-something secretaries/ nieces/ god-daughters, or a quiet lunch surrounded by disappointed tourists (especially the Japanese ones), then do go.
The food is not bad, but it's not great. The sushi is average. The service is pretty good. The atmosphere is either too much or too little, depending on timing, and it's very canteen-ish. It needs ripping down and starting again.
Go here if you're craving Japanese food and you don't have time to go anywhere else. But Zuma is just down the road (not a recommendation) and Roka isn't far (a recommendation).
PS don't go if you're really hungry