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I built one a couple of years ago, mainly in PHP and MySQL, took longer then expected but was a nice feeling of accomplishment when finished. It ended up though, that we only really use it on ecommerce sites now. I guess the advantage of using it is that we can do what we like with it, so it's always highly customised to the needs of the client. No two sites have it exactly the same.
We do use things like Wordpress quite a bit. The thing is that Wordpress is continuously being updated and I think it is very hard to build something anywhere near as powerful (not to mention plugins and the like).