Following up on my earlier post -- I went with Kiet at Milan Suit for a shirt and pair of trousers. I had a Jantzen shirt and pair of TaT trousers for him to copy. I went in on a Sunday and he had the items ready by Monday evening. Quality of workmanship was quite good, though the fabric I bought from him (cotton shirting and wool for trousers) was only okay. Fit was excellent on the shirt (per my request he made some deviations from the Jantzen shirt example), and close on the trousers (a bit tight in the seat). I also picked up some fabric -- linen from the Chinatown market and flannel from the fabric store just down the block from Milan Suit -- and left it with Kiet to make a shirt and two pair of trousers for me (I told him I would either ask him to mail to me in the US or pick up on a subsequent trip to Bangkok). I also picked out some other shirting fabrics from his selection and took swatches home with me. He labelled the swatches and said that I could email him to make an item from the fabric.
About a month later I was heading back to Bankgkok and emailed Kiet to go ahead and make the items from the fabric I had left him. I didn't receive a response from him, but when I showed up in Bangkok, the items were waiting for me. The shirt fit great. The trousers fit better than the first pair but were still a bit tight in the seat.
All in all, I'm very pleased with my experience and would recommend Kiet for shirts and trousers. Obviously, it helps if you have an existing item that he can copy. Also, if you have the patience to search for your own fabric, you can get better quality than what Kiet has on offer (though the finished product will end up costing you more).