I just received my first order from Yunnan Sourcing (the Chinese version of the site, they also have a US version with less stock) and so far am pretty happy with it.
The shipment arrived well within the time frame with no issue, in fact it was on the earlier end. While the box was slightly dented it was wrapped thoroughly in a mountain of bubble wrap so nothing was damaged. It was fun to track the package as it crossed the Pacific and most of the US.
I ordered a few black teas, two oolongs, and one pu-erh, along with a gaiwan and a small teapot. The teapot was only $13; for the price it is a good little pot, although it is by no means high quality. YS definitely has higher quality pots available and I suspect their pots scale very nicely with price. The gaiwan is very nice and close to the color on the website (blue celadon). The black teas are pretty much what I expected - rich and delicious. The leaves are completely whole, which compared to many black teas I found pleasantly surprising (even other Yunnan blacks from other companies such as Adagio the leaves were partial or damaged, without the stems attached). The oolong I tried I had some trouble with - despite brewing it very carefully in the gaiwan with ~80 C water it was still very bitter with only a small trace of the floral notes I expected. It was especially odd because it wasn't an astringent bitter but more of a cranberry bitter (without the sour... I'm not sure how to describe it! It wasn't pleasant). I'm going to play a lot with that tea this weekend because I suspect it was something I did wrong, not the fault of the tea! The pu-erh I have not had a chance to try.
Compared to other sites I have looked at the prices are very fair, even given the high shipping cost. The quality of the tea is high. It is single-origin. You do have to be careful at the amounts you are looking at when you are looking at price. Loose leaf teas default to either 50 g (1.7 oz) or 100 g (3.5 oz); pu-erh comes in 357g cakes by default (but smaller cakes are available for some). So if you sort by price some teas can look cheaper but it is actually just that they come in a smaller amount. It can be tricky to find teas that have the 25 g samplers but there are a lot of those available as well.
Look at the shipping options carefully, especially if you are ordering from China. Because my package was less than 2 kg, I was able to go EMS, which was slightly cheaper than the other options. If you go the cheapest shipping option, YS states that they will not be able to help you if any items are damaged. For tea alone this is not a big deal as tea isn't horribly ruined if it gets crushed a bit (although the full-size leaves are nice), but for teaware you definitely want to go for the higher priced option that treats the packages better.