I found this very elegant way of doing it and it solves one of the things that has always bothered me about KVO which is the large "switch statement" that KVO adds with observeValueForKeyPath. KVO+Blocks is very cool, it eliminates that "switch statement" and it automatically handles removal of the observer so no need to call removeObserver when using ARC (if you are doing your own memory management then I think you need to call removeObserverWithBlockToken, although I have not tried it).
One thing to be careful of, but is true of blocks in general, is if you reference self in your block. You need to do this:
__block blockSelf = self;
if you don't you will end up with a retain cycle. (see Retain cycle on `self` with blocks for more details).
One other thing, it works with ARC if you put -fno-objc-arc on the file. (See this for details How can I disable ARC for a single file in a project?)
I hope that Apple adds something like this to the SDK.
__block doesn't break the retain cycle. You must use __weak
KVO does not force you to use a large switch statement. If you're doing it that way, you may want to reexamine your implementation.