I just enjoy vim, and I enjoy customizing vim. Customization is a hobby I do on my own time, so I don't need any of these "you get slowed down having to customize" arguments.

I work mostly in ruby and the vim ruby experience is great. I have spent a year watching coworkers work in RubyMine and there isn't much it provides that I can't do (literally) just as well in vim. In fact the navigation tools are superior in vim as far as I'm concerned. RubyMine does have better project wide refactoring support, but with ruby being a dynamic language, you still have to go and check every file that changed to make sure it did the right thing anyway.