When our Cisco rep told us ACS was going away, and ISE was the replacement, we pushed back because ISE couldn't support TACACS. I've been told ISE 2.0 has been released (but that training won't be available for it until perhaps Q2 or Q3 2016).
My expectation is that Cisco will think strongly about following through with retiring ACS if ISE really can do all of ACS's jobs.