"Rainy day fund"?

I have been building a "rainy day fund" for a while now and currently have 4 months of operating expenses saved.

I have been told by some that this might not be the best use of my $ and that it may be better to put the money back into the business immediately.

I am weary of not having a real backup plan with the current state of the economy but am interested in hearing your opinions on reserve funds vs. putting the money to work right away.