I've found that you can write reasonably complex GUI applications using the current Apple API documents for reference and for the most part get the results you expect.
The trouble is the few times when it doesn't behave as expected. I tried porting a paint program I had once written for OS X to GNUstep. Everything was going swimmingly until I ran into a behaviour of the OS X NSImageRep caching mechanism that wasn't replicated. Unfortunately, the program more or less depended upon that behavior, so coding around it wouldn't have been trivial. I know where in GNUstep the issue lies but I just haven't had the time to have a go at fixing it, so for the time being, I remain stumped.
