I don't know about you, but I think 4 months is just way too long to expect someone to stick with it. I'm not going to invest months of time into a tool with no measurable benefit.
I used to think like that, then I realized, I am going to be editing text for at least the next 20-30 years (if not longer). I get the feeling vi(m) (and emacs) will be around then, so it's an investment worth making.
It only took a few weeks to reach the efficiency I had with TextMate, and every week since I've become more and more efficient. I haven't hit the point of diminishing returns in learning about my editor, and judging by other vim users I've seen, I probably won't for a long, long time.
If I was 100% sure vim was 'the answer', it would be worth spending 4 months on it.
But maybe emacs is the answer. Or eclipse. Or sublime text. Or Visual Studio.
Now, to give all these editors a fair shot, I'm planning on being considerably less efficient for well over a year (if none of these editors really suited me).
I don't mind other people choosing to use it, but I got more productive in 30 minutes of Sublime than 3 weeks of vim.