I like many of the points of the previous post. I think I would point out the essence of Constitutionality in the ACLU. The idea that an advocacy group is committed to the principles and ideas of the Constitution is quite persuasive. I agree that there are times when this zealous defense challenges common sense, but in the larger scheme, especially in the last 5 to 7 years, I have come around on why the ACLU is needed. This group has been one of the most fervent opponents to elements of the Patriot Act, and aspects of post 9/11 law enforcement techniques. In the days following the events of September 11, the American public was told to "embrace the dark side" in fighting the war on terror. However, I think that reflection and historical distance has allowed us to see that there were moments when we actually explored our own "dark side." This is not what the Constitution was designed to support and the ACLU has been quite vehement in ensuring that we, as a nation, commit ourselves to fighting the war on terror without the sacrifice of our ideals. The group stands true to this because it is a reflection of our own history and sense of self, but also because if we betray our legacy, we provide enough ammunition and hypocrisy to those who wish harm to us. In some odd ways, the ACLU and groups like them help to enhance the best aspects of American History with their zealous defense of the Constitution.
The ACLU is one of the most reviled institutions in the US, especially among conservatives. For my part, I'm pretty ambivalent about it.
On the one hand, the ACLU is committed to fighting for our civil liberties. Many of the causes it takes on are very important (of course all of this is in my opinion -- no one can objectively say which of their causes are important and which aren't).
However, it seems to me that, like most advocacy groups, it goes too far at times. It seems to me that it defends fringe activities on the grounds that any erosion of civil liberties is a step on a slippery slope to the denial of all civil liberties. I don't like it when the NRA opposes things like the ban on assault rifles and I don't like it when the ACLU fights for the right of an artist to use a government grant to take a picture of a crucifix in a container of urine.
Overall, I'd say that the ACLU is like any other advocacy group -- I agree with much of what they do, but I think they go too far at times.
The ACLU is slightly extremist and arguably favors militant methods, as their controversial action does help push their cause.