With Burr grinders your coffee beans more evenly ground. With choppers, some ends up powder, some ends up kinda big pieces.
Burr grinders are usually able to make a finer grind. This is very important for espresso and turkish coffee. Not so important for drip.
Burr grinders create less heat in the beans when grinding, resulting in less chemical change in the coffee beans. Some people claim this results in better flavor.
Burr grinders cost at least $50-$75 for a cheap one. A perfectly good krups blade grinder costs $20.
This comment was edited on Feb 5, 14:29.