We went for an evening meal at the Bamboo Asian fusion restaurant (there are different bits of Zen Garden open in the evening and during the day).
The food was good - we had a couple of the soups (generous helpings), spring rolls, dumplings and other starters, and then sushi and pad Thai for mains.
BUT I have to say its pretty overpriced, especially the sushi - KES 1000 and upwards for the very plainest rolls, with the platters at KES 3000! I would go to Onami in Westlands instead if you want sushi - its pricey too, but not this pricey... Even a bowl of soup here sets you back over KES 800 ($10). For Nairobi, that is expensive.
That said, the selection is good, so if you are in a larger party, which we were, almost everyone can find something on the menu they like.