Positive is the built-in health monitoring & recovery, which replaces faulty servers of the cluster automatically. Also good is that the overall pricing is similar to a normal EC2 instance, but tuned for 64bit and a high I/O capacity.
Not so good is that AWS still does not seem to learn from their recent disasters, because an ElastiCache cluster is always just hosted in one Availability Zone. When that AZ fails, your complete cluster goes down. Multi-AZ need to be manually designed in the application code which is actually a very poor solution. But AWS iterates their services often and I am sure that Multi-AZ will be available soon for ElastiCache.
Watch the official tutorial video and skip to 4:50:00 to see the ElastiCloud in action.