I just wanted to clarify something but you can get free TACACS software for Unix so cost of ACS need not be a con. CCO link about the freeware Unix version below along with some config stuff:
However some cons could include: increased administration, need to possibly acquire/purchase a new server.
On the plus side, it does offer total accountability and a solid audit trail on your network. If you company is into knowing who did what and when, TACACS is for you. Buckwheet mentioned the 3 AAA's. Togive more detail about them:
* Authentication: The process of validating the claimed identity of an end user or a device, such as a host, server, switch, router, and so on.
* Authorization: The act of granting access rights to a user, groups of users, system, or a process.
* Accounting: The methods to establish who, or what, performed a certain action, such as tracking user connection and logging system users.
The following is a good Cisco link about how to setup TACACS: