First off, all news stations have political commentators who maintain fairly extreme views ... Bill O'riley and Keith Olberman are (probably) the obvious examples, and whether you think one is more extreme than the other is probably more a reflection on your own personal bias than anything else.
When it comes to their actual news programs, while there are still problems with bias, most of the networks do a decent job of remaining somewhat professional and there is a major problem when they choose to (or are prevented from) being able to adequately investigate/report on a news story. If George W. Bush prevented CNN or MSNBC from asking questions in a press conference I'm sure many people would have a different view of the action, but it is equally wrong in either case ... One of the ways that we prevent corruption in our system is to ensure that our elected officials can be watched and their actions can be questioned by everyone, including people who will never accept any justification.