The Cisco ACE would typically (AFAIK) be deployed in an active-passive pair, so if one goes down, the other should take over. This is independent of DNS (DNS would just be another type of service being provided through the ACE). This would be suitable if both DNS servers are in the same data-center (or possibly metropolitan data-center).
One thing to bear in mind about an ACE (I imagine this will depend on your deployment), but if you need to know the client IP (perhaps you are doing split-DNS with different client views), then you will lose that visibility, and you would need to point internal and external clients to different (load-balanced/shared/redundant) servers (which is closer to best-practice for DNS anyway, as you'd want internal (recursing, caching) servers seperate from your external (authoritative) DNS servers.
Dynamic DNS updates will also be something to consider (where is the state in your cluster?)