How does proportional representation compare to the American system?

In a proportional representation system, parties generally
get to be represented in the legislature in proportion to the amount of the vote they
receive.  In other words, a party that got 25% of the vote in a US election would get 0%
of the representation.  In countries like Germany that use PR, that party would get
around 25% of the seats in the Parliament.

Some argue that
this system is fairer than the US system.  It allows more parties to enter politics and
thereby lets more people have their voices heard.  In the US, a large minority might not
be heard because it cannot defeat the major parties.  In a PR system, it would be
heard.

The flip side to this is that a PR system is more
prone to extremism and parties that concentrate only on single issues.  A party can get
into the government and "cause trouble" simply by having strong support on a single
issue.  Parties in PR, unlike those in the US, do not have to appeal to a broad range of
people and so they do not encourage compromise.