By dramatically reducing the time it takes to develop and implement an application, Force.com completely changes the old Build vs Buy dilemma.  With Force.com, it is becoming much more attractive  to build a solution to meet needs your specific needs instead of buying a package and trying to make it work for your environment. 

We are seeing this all the time now - companies that before would never have even considered writing their own solution are now shunning the packaged software route and building from scratch.  A sign of the times...