Using a Cisco ASA Firewall vs Using a Cisco edge router with several access-lists

This is mainly an information gathering issue and would appreciate some expert opinions. Most of the time, I have seen offices uses dedicated "firewall" systems such as the Cisco ASA or PIX to protect their network from unwanted intruders. However, I am working at a new organization that the prior admin put in place a Cisco router with detailed access-lists instead of dedicated firewall to protect the network from the outside. I was wondering that the pros and cons are for using a dedicated firewall system like the Cisco ASA vs using an edge router with access-lists? Thanks in advance.