Here is why not to buy the stock (from the Google Prospectus):
The principal purposes of this offering are to obtain additional capital, to create a public market for our common stock and to facilitate our future access to the public equity markets.
We currently have no specific plans for the use of the net proceeds of this offering. We anticipate that we will use the net proceeds received by us from this offering for general corporate purposes, including working capital. In addition, we may use a portion of the proceeds of this offering for acquisitions of complementary businesses, technologies or other assets. We have no current agreements or commitments with respect to any material acquisitions.
Pending such uses, we plan to invest the net proceeds in highly liquid, investment grade securities.
They have no concrete plans for what they want to do with your money, other than to give the initial investors of Google an opportunity to take it.
Just what is Google going to do with 1.6 billion that they can't do with their own private funds (they have over a half-billion liquid right now)?