I ordered these 3 filters from PQ23. See MO here. I am rather impressed with its build quality and it doesn't feel or look cheap at all (though the price is much cheaper than Lee or Cokin). Here's my unscientific test with the 3 filters I've bought. Please excuse the dust spots because I haven't cleaned my sensor for the longest time.
Graduated Grey (ND). Gives a slight yellow cast but can be easily fixed in post production unlike Cokin which can be quite a pain to remove. My Lee ND Grad still gives the most neutral gray but its costs 13 times more.
I also purchased the filter holder and ring and I have sad news for full frame users though. At 17mm the sides of the filter holder can be seen very clearly. Only at 20mm then you'll get no vignetting. I am using a 17-40L + 1Ds btw.
Overall, I am pretty pleased with the Tianya filters. Thanks to PQ23 for the MO and bringing in good products at decent prices.
More testing will be updated here once I go out and do a more extensive test. Watch this space for updates and hope that helps all who have any doubts about the Tianya filters.