I would say that if Redis is an effective caching solution for you, then ElasticCache will work for you - you're simply paying AWS to manage the back end and plumbing for you. Performance may be marginally slower - you have to have a DNS lookup for requests, vs having redis running in a VPC where you can access a private IP address directly - but even accessing it from an EC2 instance should resolve the public DNS name to the internal private IP. And of course you can launch your EC node in your VPC.
There are some complications when running a memcached cluster - you will need to use the amazon client to make sure your code connects to the correct node - but I do not believe as of Dec 2013 that this is needed for redis.
If you're implementing a queue on top of redis, have you looked at SQS to see if it will work for you?