I have only personally had experience with two different tailors for suits - Hahn's and Savile Row, both in Itaewon.
I was very unimpressed with Hahn's. The jacket came out with a horrible cut - the chest floated far too high off the body and when buttoned there was almost no spread at the bottom of the jacket. Additionally, when I asked the proprietor to address these concerns he just jury-rigged a subpar solution for the chest and ignored the other issue as just being part of the style in which he had made it. Overall seemed like poor craftsmanship and service. Additionally, he tried to peddle me some of that cheap Korean 180's cloth and claimed it was just as good/strong as anything else. Savile Row immediately told me they didn't stock any of that "fake" stuff.
Savile Row, on the other hand, I have been extremely happy with and have since used it for half a dozen suits and an overcoat. The owner has since the beginning offered very personalized and caring service, and the quality of the tailoring has been excellent. We went through a few fittings at first but they worked with me to get an excellent cut which they have since been able to reproduce on any of my orders. They have also never once hesitated to fully address any concerns or alterations.
On the topic of shirts - you should first be aware that most of the tailors do not make their shirts in house. Both Hahn's and Savile Row outsource their shirts to basically the same external bulk tailor. I have gotten shirts from both of them and they were both lacking. Because of this I would not suggest getting shirts done by either of them (or by most any other tailor for that matter because it will be the same situation). For shirts I would suggest Hamilton. They are particularly focused on shirts and many people I know have testified to their quality. There is a Hamilton store in Itaewon (down closer to the subway station on the side of the road opposite Hamilton Hotel), and a store in Myeong-dong in the middle of the central shopping area.
Edited by Accountancy - 8/27/11 at 11:21pm