For me the advantage of Emacs over a set of IDEs is its ability to support multiple languages very well within a single unified environment. In a typical day I'm writing Common Lisp, C++, XML, XSLT, shell scripts, documentation. The only time I'm not working in Emacs is when I switch to Eclipse for the odd Java project, because that's what everyone else uses in my department.
Have a I spent a lot of time customizing my environment? Absolutely. But it fits me like a well oiled leather glove and makes me incredibly productive. I don't need to learn separate environments for Lisp, C++, XML...
A lot of this is personal preference and familiarity. When I sit down with Visual Studio or XCode I'm completely lost and feel like I'm wearing shackles.
I love a good IDE (RubyMines, for example), and in general a well-engineered IDE designed from the ground up for your language and development stack is going to be superior to emacs or vim.
However, I'd feel crippled overall as a developer if I didn't know one of those two (vim in my case). For one thing, a lot of the best IDEs are not free and some are very pricey. I'm not going to go out on a huge spending spree buying a bunch of IDEs just for little experiments I want to do in my spare time. More importantly, though, it is quite often at work that I need to edit code in a variety of languages, and it is simply more productive to be able to edit the code in one editor rather than having to fire up multiple IDEs. Not to mention the need to occasionally make quick edits from within a terminal.
Using vim frequently also keeps me up to speed on terminal commands for lots of modern tools. When I was in college I used IDEs almost exclusively, and whenever I found myself needing to do something from the terminal, my God it sucked. I was crippled. Now I feel more like a jack of all trades who can be productive in mostly any environment.
Have a I spent a lot of time customizing my environment? Absolutely. But it fits me like a well oiled leather glove and makes me incredibly productive. I don't need to learn separate environments for Lisp, C++, XML...
A lot of this is personal preference and familiarity. When I sit down with Visual Studio or XCode I'm completely lost and feel like I'm wearing shackles.