[phil]

I've subclassed NSManagedObject and added instance variables and custom accessors literally every time I've written Core Data code. It's usually not a problem -- yes, you do have to do a couple things to preserve the underlying behavior.

What problems have you had?

[pifflesnort]

I've found the 'couple things' to introduce enough overhead, that when coupled with the other downsides, Core Data hasn't saved me time, reduced LoC, provided any performance advantages, or improved code quality.