Gameplay's pretty solid. Could use a bit of alteration of the randomization algorithm such that long strings without a piece of the player's color are avoided, just to make things less frustrating to the player. (Simple example: store the last 8 dropped pieces, and if at any time that list has no pieces of the player's current color just force the next drop to be that player's color instead of random.) Otherwise it's fun.
The scoring system makes no sense, though, but I guess that's not much of an issue without an online scoreboard. Still, it does provide serious progression counter-incentives for many players.
(Those of you who are trying to use the mouse instead of the keyboard: cursor controls losing focus when the mouse goes outside the screen have been a problem since Flash games first started. Just click onscreen and don't let go of the button, so the game won't lose focus if the mouse goes outside.)