Originally Posted by cyccommute

Then you've come to the wrong place. All you will get here is a bunch of lame jokes from the kind of person who used to sit in the back of the class and crack comments that they and their 4 loser friends thought were funny.

First, you don't have cantilevers on your bike...not if you have disc tabs on the frame. You probably have linear pull brakes, aka v-brakes, which is a plus because you won't have to buy levers for cable actuated discs. Next, as others have said, don't go cheap. Cheap discs aren't any better than cheap linear brakes and are worse than a set of good linear pull brakes. For cost of the brakes alone, expect to spend between $100 and $130 for a full set. Then you will also need a set of wheels. You can get a set of wheels anywhere from $90 (cheap ones) to $250 (pretty good ones) and up. You are looking at $190 to $350 plus shipping for the parts. It might be worth $190 but at $350 that's a lot of money to throw at a $500 bike to make it into a $600 bike (retail price of a Rockhopper and a RH disc). Is it worth it? That's up to you.

As to whether or not it's an improvement? Someone said it's like night and day. It's not. It's more like the difference between a sunny day and a cloudy day. The v-brakes will stop you in just about any kind of conditions that the Rockhopper was built for. The discs will stop you in the same conditions and will do a bit better if you ride in wet conditions. Worth $200? I'd save money for a new bike first.