Outside of Thailand, this is about as good as it gets food-wise. Went with my husband, who never seems to get enough chili, so he was quite happy that the chef really did make everything he ordered appropriately "Thai spicy." My only tiny quibble is that the curries are a little thicker than what you would get in Thailand, but their taste and composition more than make up for that. Try one of the duck dishes, they're delicious. Big kudos to the wait staff - they are all extremely welcoming and attentive, but don't hover over you at your table. They also do a surprisingly delicious fried ice cream for dessert; I've had it at other restaurants and have never been impressed, but this was recommended to me by one of the waiters and it was fantastic as promised.
As some reviewers have pointed out, Thai Chi is perhaps a little pricey, but considering the quality of ingredients, extremely well trained staff, and its location in one of Nairobi's nicest hotels, it is definitely worth the money.