I bought OPI Design Series Mystery, brought it home, swatched it, and realized that it is really close to China Glaze's Midtown Magic (MM) from fall, 2011. They aren't dupes, but they are so close that I feel that owning both is unnecessary. Both have a warm, burgundy-brown base, although Mystery is slightly darker than MM. They both have flecks suspended in the base, but there, they diverge. The flecks in MM are tiny, round, regularly shaped, and firey. The flecks in Mystery are oblong, slightly irregular, and a cool gold tone. The formulas are equally good for both polishes. In the end, I suppose it comes down to minutia, since they are very, very close. I'm trading one out, but I'm not telling which one. I don't want to play favorites.
In the photos: OPI is on my fingers; China Glaze is on my Thumb.
Below: Close up of OPI.
Below on thumb: China Glaze Midtown Magic
Below: Left, China Glaze. Right, OPI.