Recs.
Updated
SpecsUpdate
Pros
Pro Built-in terminal/run tools
WebStorm (and really all of the IntelliJ IDEs) support the plugins throughout their plugin ecosystem which leaves you with 100s of tools to handle your automation tasks. There is a wide range of build-related plugins that help you by having pre-defined commands to execute with the click of a button. Out of any other IDE, WebStorm has by far the most coverage when it comes to tools for your development workflow.
Pro Built-in web server
WebStorm has a built-in webserver that allows you to run projects from http://localhost:63342/ProjectName.
Pro Intelligent code completion
WebStorm has two types of autocompletion: structural completion and word expansion.
Structural autocompletion makes predictions based on its understanding of JavaScript objects, while the latter tries to predict the word currently being typed based on previously typed words. Word expansion also works in comments and docstrings and it's similar to vim's omnicompletion.
Both types of autocompletion work extremely well, have little to no problems and are quite fast even when loading suggestions on the go.
Pro Integrated code quality tools
WebStorm comes bundled with JSHint and JSLint. JSCS, ESLint, and Closure Linter can be installed via npm. They register as inspections and are customizable through IDE settings. They run automatically and will highlight potential issues. Pressing alt+enter on an issue will allow the user to view suggested fixes.
Pro Ability to base hot keys on previously used IDEs
When you install WebStorm, it allows you to choose from other IDE's for it to base it's hot keys on. So if you are switching from another IDE it makes it a very easy transition and productivity is not adversely affected by having to learn new hot keys.
Pro Interactive theme (color scheme) editor
Makes adjusting an existing or creating new themes a breeze. Especially due to things like inheritance, as well as easily exporting/distributing/importing the color settings which really only store where on deviates from the defaults (thus the resulting files are very small and relatively human-readable).
Cons
Con Slow performance occasionally
WebStorm can sometimes choke all cores of the CPU. There are numerous reports of high CPU usage.
Con Non-native filesystem causes issues
The Java wrapper around the filesystem doesn't actively watch for file changes (by, for example, using the fsevents api on OS X), and as a result can easily become desynchronised from the actual filesystem.
If you have an external tool acting on your project (such as, for example, a gulp task or a third-party Git client), what you see in the file browser or in open tabs becomes out of date. There's a feature in the context-menu for manually synchronising directories with their real filesystem equivalent, but this shouldn't be necessary, is annoying to do, and usually you remember to do that after you've been trying to track down a bug on a line of javascript that Webstorm says doesn't exist for the last two hours.
It should be noted though that this is easily remedied by going to File/Settings/System Settings and checking the "Synchronize Files on frame or editor tab activation" option. It's also recommended more explicitly representing your workflow within WebStorm itself. Most external tools/tasks can be handled of to WebStorm. If it's not in a plugin then you can handle it with the File Watchers.