IDEA places an emphasis in safe refactoring, offering a [variety of features](https://www.jetbrains.com/idea/features/refactoring.html) to make this possible for a variety of languages. These features include safe delete, type migration and replacing method code duplicates.
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Pro Extensive support for programming languages
SlickEdit supports over 50 programming languages on nine platforms.
Write custom macro commands, functions, dialogs and tool windows.
Windows, Linux, Mac OS X, AIX, HP-UX, Solaris SPARC, Solaris x86
Pro Symbol analysis support
There are powerful symbol analysis features in SlickEdit, including context tagging and references.
Pro Extensive configuration options
Pro Integrated debuggers for multiple languages
Integrated debuggers for GNU C++, Java, Python, Perl, Ruby, and PHP.
Pro Built-in beautifier
The beautifier formats code as you type to help improve readability and consistency.
Pro Compiler tools
Pro Over 13 emulations
Choose from fifteen keyboard emulations, containing the key bindings and behaviors necessary to emulate other editors (e.g., CUA, Vim, GNU Emacs, etc.)
Pro Portable mode
Possibility to set up a portable installation, to run on a USB drive for example.
Pro Easy access to XCode projects
SlickEdit opens XCode projects with no conversions needed.
Pro Third party tool integration
Pro Easy access to Visual Studio workspace
SlickEdit opens Visual Studio workspace with no conversions needed.
Pro Popular version control system
Pro Smart refactorings
IDEA places an emphasis in safe refactoring, offering a variety of features to make this possible for a variety of languages.
These features include safe delete, type migration and replacing method code duplicates.
Pro Fast and smart contextual assistance
Uses a fast indexing technique to provide contextual hints (auto-completion, available object members, import suggestions).
On-the-fly code analysis to detect errors and propose refactorization.
Pro Android support, JavaEE support, etc
A very complete development environment support.
Pro Support for many languages
IntelliJ supports many languages besides Java, some of these are: golang, Scala, Clojure, Groovy, Bash, etc.
Pro Lots of plugins
Many plugins are available for almost any task a developer may need to cover. Plugins are developed by Jetbrains themselves or by 3rd parties through the SDK available for writing them.
Pro Intuitive and slick UI
IDEA has a clean, intuitive interface with some customization available (such as the Darcula theme).
Pro Stable and robust
IntelliJ IDEA hardly ever crashes or has any issues that plague other Java IDEs like file corruption or slowness.
Pro Clear and detailed documentation
The documentation is exhaustive, easy to navigate, and clearly worded.
Pro Free version available
There is a free community edition (open source) and an ultimate edition, which you can compare here.
The ultimate edition is available for free for one year for students but must be registered through an .edu e-mail account.
Pro Very powerful debugger
With ability to step into a certain part of a large method invocation (Shift+F7), drop frame, executing code snippets, showing method return values, etc.
Pro Many convenient features
These simplify the daily work, e.g. copy/cut a whole line without the need to select it.
Pro Gradle support
Pro Buit-in Git support
Con It's kinda slow
If you have a very large project or tag database, it can hang the UI
Con Uses a lot of RAM
Con Slow startup
Startup can be slow deepending on system configuration
Con Standard hotkeys behave differently
Seems like hotkeys assignment in Idea has no logical consistency.
Like «F3» is usually next match, «Ctrl+W» - close tab, etc — they map to some different action by default.
There is a good effort in making the IDE friendly for immigrants from other products: there are options to use hotkeys from Eclipse, and even emacs. But these mappings are very incomplete. And help pages do not take this remapping into account, rather mentioning the standard hotkeys.
So, people coming from other IDEs/editors are doomed to using mouse and context menus (which are rather big and complex).
Con Bugs are not solved as often as they should
They are more interested in adding new features or issuing new versions than solving bugs.
Con Cannot open multiple projects in the same window
Con Built with closed source components
The version with full features is not opensource. Parts of the code are under apache licence though.
Con Lack of plugins
IntelliJ supports a very small amount of plugins. Although these are 'quality approved', many features are missing and can't be implemented because of that.