According to the Wall Street Journal blog, it's sold 3.3 million tickets in 5 days. Since it was a record-breaking weekend in general, I'd agree with Mark Russell's guess that the hot weather has helped it along (says me, the guy enjoying a sunny, 21 degree afternoon in Canada).
It's currently the fastest selling film in Korean box office history, selling a million tickets in less than two days.
I also saw Stoker finally, on the plane ride to Canada yesterday. While beautifully shot, the story was lacking (it wasn't Park Chan-wook's script, though his two previous films, scripted by him were also lacking). I'm not in a hurry to see Kim Ji-woon's Last Stand (the other 'first English-language film' by a Korean director this year) but can easily say that Bong Joon-ho's first international effort is the strongest of the three, and certainly more mainstream - an action film with brains, so to speak (complete with his usual social commentary).
Not much more to say than 'Just go see it.' It's out now in Korea (especially at CGV, unsurprising considering CJ Entertainment helped fund the film) and is well, well worth watching. I've written about it before, and after I watched it I realized I'd forgotten most of the details of the trailer I posted awhile back. This is a good thing. The surprises made the movie that much better. So go watch it.