What are some of the advantages and disadvantages of using a mainstream news source?

I assume when you say "mainstream," you are referring to a major national TV or radio network (like ABC, NBC or CBS, and on radio, NPR), or a major national cable channel like CNN, Fox, and MSNBC. It should first be noted that some people believe the myth that the "mainstream media" are all liberal (except for Fox), but in reality, the mainstream media are corporate. More will be said about that later in this answer.

The primary advantage to the mainstream news media is that it has a global scope, which means that reporters can cover stories from all over the world. Mainstream news is also perceived as more professional and reliable than what you may find on blogs: mainstream news reporters have to do some fact-checking, and their work is also supervised by editors. And mainstream media have some high-profile star reporters, both in print and on TV & radio, who have Excellent contacts and are very well-known for getting to the story and telling it thoroughly and accurately.

But a major disadvantage to any corporate medium is that the news can be influenced by sponsors or by whoever has political power. A network might hesitate to do an investigative report about one of its biggest advertisers, or it might shy away from saying negative things about a very influential political figure. Corporate media often avoid criticizing capitalism or criticizing big business, since the owners of the networks would not appreciate being the subject of such criticism (even if it is deserved). The other problem with the mainstream media is that more of them are having a hard time separating commentary from news, especially on cable. Some mainstream networks do lean liberal, as MSNBC does, or they may lean conservative, as Fox does; many, especially the broadcast networks like ABC/NBC/CBS, try to be centrist, and so does CNN on cable. But no network or channel or publication is entirely neutral, since the perspectives of those who write and report will often get into the way the news is covered.