I have personally bought a few lenses from Tin Cheung (HK) online and so far no problem. But make sure you tell them you want a spesified invoice or they might not give you one, you'll need it for the 1 year international warranty to be valid. They will ship their lenses by FedEx express so usually they arrive at your doorstep the next day.
The last lenses I bought from them was a Sigma APO 120-400mm F4.5-5.6 DG OS HSM, a Sigma APO 50-150mm F2.8 II EX DC HSM and a Sigma 30mm F1.4 EX DC HSM. They are all excellent lenses for the price. I did consider the Sigma 150-500 APO OS HSM but chose the Sigma APO 120-400mm F4.5-5.6 DG OS HSM in the end because it's more compact and weigh a bit less.
The Sigma APO 50-150mm F2.8 II EX DC HSM and the Sigma 30mm F1.4 EX DC HSM were both not fucusing correctly (when using AF) when I received them so I had to bring them down to the Sigma service center to have them calibrated. That took about 3 hours and when I picked them up the technician went through the test shots before and after with me and explained how the calibration process were done. Excellent service! And I told them the lenses were bough on-line from Hong Kong, they didn't even ask to see a receipt or a warranty certificate. After calibration both lenses focus perfectly!
So I have no problem recommending buying Sigma lenses from Tin Cheung (HK) online.
But do take note that FedEx will collect the Singapore GST so you can't escape that.
