8 years, $1 Bn ERP System – Worth It?

In a related post I copied an WSJ article that discusses a very large ERP system implementation at Dow Chemical.  The system processes ~ 7 million transactions / day and counts > 16,000 unique daily users.

Note: I was a member of this implementation team (i.e. a very small portion of the $1Bn investment paid my salary).

Questions:

1. Why would an implementation take this long and cost so much?
2. Given the information shared – why would Dow Chemical company approve such a large investment?