The Cocoa API is pretty well thought out, generally well-documented, and pretty well-engineered.
I agree, their APIs are generally very nice. But the last few releases of OS X haven't been great. Multi-monitor support will finally be fixed in 10.9, after having been broken in 10.7 and 10.8. Also, recent releases have become very slow. A 2009 Mac Mini running 10.8 is unbearably slow, while the same machine is on 10.6 is almost as fast as a recent machine machine with a SSD on 10.8.