Its essentially branded monodevelop with bundled addins(or is mono develop now unbranded xamarin?). In any case, it has improved dramatically over the past couple years. I write services/daemons in it and find the C# functionality to be fantastic now. F# is coming along too.
Xamarin Studio is definitely rough around the edges but for the most part it seems to get the job done. For example, I've had issues where the update dialog steals the modal focus from the source control dialog(s) and that's basically the end of all interaction with the app until I restarted it.