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MonoDevelop is free to download and use.
Con The MonoDevelop version that ships with Unity is several versions behind
The version of MonoDevelop that ships with Unity is several versions behind the main MonoDevelop branch. It also gets updated very rarely so any annoying bugs that it may have take a lot of time to get fixed.
Con Bad formatting
MonoDevelop doesn't offer much in terms of autocompletion and code formatting. Most of the time the automatic formatting that MonoDevelop does is annoying and not really compliant with C# guidelines.