They are genarally good as long as there are enough of them to represent the views of all the people. But the system we have in the U.S. today, were there are only two parties, which are simply the right and left wings of the same fundamental ideology, cannot support the views of the people-they are not diverse enough politically.

A system were anyone can start a party based on new ideas and have that party recognized is a system of true democracy, anything else is not.