I only got invested in Vim at work, where I actually had to use a text editor for a significant portion of my day (actually, ViEmu, at the time). For university work, I never used Vim for quite long enough for it to really sink in.
Sure, you lose most of your productivity for a few days when switching text editors. So, I guess you should not switch editors when time is at a premium and deadlines are piling up. But you certainly need to have a good amount of time where you use the software full time, if only to build those all-important habits.
By the way, this is how I learned Vim, Emacs, and (shockingly) touch typing. It worked well for me.
For those who don't use vim, switch right now (unless you're at work.) If you want to be productive, you'll have to learn to love the home row.
As an aside, it might be worth it to learn a couple of emacs movement commands as well if you're using OS X, as you'll be able to navigate textboxes without leaving the home row as well!
Let's say you delivered packages using a horse and buggy, and people started talking about this new fangled "automobile" thing. Sure, you could try to dive into it and use an automobile for your next delivery. And when you crash the thing and your delivery isn't made, you get pissed off and say "I'm never using an automobile again!"
Or.. you could keep using your horse and buggy at work, and start learning the automobile in your free time. Then, once you've mastered the automobile, start using it at work.
But I'd disagree with the advice of learning vim a little bit at a time. It may be necessary to do that if you're trying to learn vim at work, but most people will acquire bad habits that way and learn to use vim in a watered down way. You've now tied some reins to the steering wheel and are sitting on the automobile's roof, trying to reach in the window and whip the gas pedal.