Do You think that Asterisk can be a feasible solution for enterprise firms?

Offcourse it is. You can estimate its importance by knowing the fact that even Google and ebay are using Asterisk.

And it is ture that you can expand it as much as you want and still its free...

Asterisk is not only Used at Large Enterprises but used in Large Scale Carrier envrionment as well. It can by scaled to any Level you need.

Astersik still a free working solution but not for large scaled firms... you will need other professional and robust Voip solution.

Yes, Asterisk growing open source technology and its viable for enterprise solutions