Using replay, replayLast, and replayLazily operators on an infinite signal leads to memory leaks if you're not careful. Be sure to clean up after yourself.
One of my favorite ReactiveCocoa operators is flattenMap, and there are a two times I leverage it: incremental loading and mapping bad values to errors
Objective-C's delegate pattern isn't flexible and spreads out one particular concern into multiple areas. Here's a way to replace it using ReactiveCocoa.