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Why use css 'frameworks'?
If you are pressured for time.
If you do not know css, and don't know someone who can write it for you.
If you are not overly precious about standards etc.
I know programmers who have been really happy to use blueprint or 960, as it allows them to put together a layout on their own, without turning to a front-end developer. This is ideal for personal projects, or startups with limited resources.
If you have decent knowledge of CSS already, then presumably you have a decent library of stock layouts already, so you clearly won't need a framework.
However, if you're a beginner and just need to get something up and running, then you might turn to a framework, as it makes basic layout much simpler, and tackling browser compatibility also.
Having said all that, many frameworks out of the box do make use of some horrible class names etc. I know of some websites that have taken a framework as a starting point and then customised it with their own class and id tags. But clearly there's a bit of work involved in that rewrite too. Using something like Compass, as mentioned above, does help to get around that.
So, CSS frameworks - they can save you time, at the cost of semantics. They might also hurt your knowledge of CSS, but that is more up to how much you invest in learning the subject in general. Whether you make use of them is up to you.
I have used BluePrint and YUI but I always get frustrated with some of the names they give their id and classes.
To each their own, but I prefer doing things from scratch, but after a while you develop a process in which you will use your previous work and apply it to new projects and just make some tweaks to make the web site look the way you would like it to.
Be sure to use a good naming convention, just in case someone else down the road comes in to edit the css, then they will have a good idea what each style name is referring to.