Which tasks are more suitable to NSOperation than GCD?
Which tasks would be better suited to using NSOperation as opposed to using GCD when programming for the iPhone?
To me they seem to do the same thing. I can't see the strengths and weaknesses one has over the other.
Answer
NSOperation is built on top of GCD, so the question is more about whether you use NSOperation or pass a block directly to GCD.
An NSOperation is bulky and needs more boiler-plate codes to set it up, but it has a lot more functionality. You can create the same NSOperation subclass in various parts of your code and put them into the queue and run it.
Passing a block to GCD by e.g. dispatch_async is quick and disposable. You typically don't reuse a block anywhere else; you just set up a block which is executed only at that point of the code, passes it to the GCD or other APIs, and quickly go on.
So each has its merits.