Pretty good, I got rather addicted to it. I have a couple complaints though. One, once the prices started dropping, they never turned around and went back up. In the real stock market, half the skill is in knowing weather or not to sell when the price starts dropping. My other complaint is that there should be more depth. You should have more goals than just "Buy the mansion". If you did those two things, it would add a lot to the game
It's fun. ;D