Basically this could be the next episode of the Oceans series, except the cast is a collection Chinese and Korean movie stars. If you are not familiar with them, this movie will sometimes drag on as various actors joke around and have fun on screen, while the plot drags to a slow crawl. Then there are the occasional flashbacks to explain different characters' motivations in the present. Its often frustrating to be constantly interrupted like this in the midst of a complicated heist to rob a casino, not to mention the various double-crosses each of the many characters are planning behind the scenes.
Having said that, I'm sure the reason that The Thieves is Korea's all time box office champion is because the interactions between the star actors is much more amusing if you really know them well. There are some awesome action scenes that rival the likes of Mission Impossible, and the humor does actually work some of the time, even with subtitles. It doesn't hurt that the cast is full of eye candy as well. But overall, the film would have worked much better if there was about 30 minutes shaved off.