The Count: I just bought some yunzi stones and jujube bowls from YMI. They mistakenly sent me some cheaper bowls but when I emailed them about it they said they would send the correct bowls and let me keep the cheaper ones (probably because returning them would not be worth the postage cost). I don't expect to receive the right bowls for a few days.
I live in the UK, and it is worth noting that there were quite significant additional costs on receiving the goods. I was charged about £27 extra before I could receive them. I had already paid for the good themselves and for postage. The £27 was half import tax and half a Parcel Force fee. I think this was for the postage within the UK. All in all, the actual product accounted for less than half of what I paid. If you don't live in the US, please consider this before buying. I later realised that Sensei's Library shows that yunzi stones can be bought from within Europe. I haven't investigated this to see whether it would have been cheaper.
-- Unqualified Newcomer: I just wanted to mention my own experiences with YMI, as a resident of the United States. I have purchased a magnetic Go set from them for astoundingly little money, as well as one of their bamboo boards (3/4" thick, with 19 x19 on one side and 13 x13 on the other) and a set of their Yunzi stones, with other assorted odds and ends. I can state without any reservations at all that I am delighted with the products. The stones in particular are wonderful--although I've never played with slate and shell. Everything is packaged to survive a war, and my shipments were always received promptly. I heartily recommend them to anyone in the United States.