Ranked in these QuestionsQuestion Ranking
Pro Testing tools are handy
Pro Ctrl + T is useful look of classes, enums, interfaces, etc
Pro Refactoring tools are powerful
Extract interfaces, generate constructors, rename namespaces, create equality statements, create ToString() summaries, etc.
Pro Extremely useful when reading code written by others
ReSharper has a built-in "go to implementation" feature which takes you to the body of the implementation of a method from an interface. This makes it easy to read code written by other people simply by going over the interfaces and using this feature to quickly find the implementation of a particular method.
Con Increases build (compile) time
Adds a considerable time onto building my Xamarin Forms project.
Con It can mangle Visual Studio settings
Occasionally Visual Studio and Resharper settings can get into a tug of war. Features can disappear, for example, Intellisense can vanish, and you're often resetting options in hopes of getting yourself back to a working IDE.
Con Licencing cost and T&Cs
The license T&Cs are somewhat harsh. The yearly license gets you a year's worth of updates, but you don't get major version release within that window. Instead, you're downgraded to the previous major version that you bought the license on.
Con Performance leaves something to be desired
The performance has been an issue for years, and many developers who have tried this tool gave up because of the performance hit running it. You may often find the tool to make Visual Studio slow down to almost an intolerable level.