MonoDevelop on the Mac bothers me. Even though the apps you create can be proper (and they emphasise that fact), the IDE itself screams "this is a second-class environment" by looking and feeling nothing like a Mac app.
... which corresponds with the state of almost every IDE on OS X.
XCode, on the other hand looks great. But it's also a pretty horrible environment to work on. Pretty much zero helping writing the code, unlike any other IDE out there.