sure the 200 dollar price tag seems steep. and you can sure buy a microcontroller for a fraction of that, but the point is that it's LEGO. and along with the programmable brick you get a bunch of lego technic parts. That allows for easy and rapid development of ideas. Let's say all you want is a simple rover bot. If you were to build it from scratch, it would cost you a few more bucks, and a whole lotta time. But with Lego, it's about 5 minutes of snapping bricks together. You can try and retry things VERY quickly with a Mindstorms set.
But the RCX (the heart of the mindstorms kit) is limited somewhat. It only has 3 inputs and 3 outputs.
If you want to do some simple (and some not so simple) robotics projects then Mindstorms is a great way to get started. Then you can move onto other microcontrollers as your needs grow.
-patchy