What is the Best IDE for Magento?

That new extension is really nice.
I am wondering how good it works, I gave it to a couple of the guys at work here, who use Eclipse. I say, that this is subjective. Its really about what you feel most comfortable working in. Each interface is different and everyone has an opinion on what one is better. In the end, try them all out, and use whatever one you are able to easily use its features and navigate the system.
Related post:

PHPStorm is the best in my opinion

(however NetBeans also works good with Magento, probably Eclipse and Zend Studio work good.)

There are main features of PHPStorm.

Code Completion
Strong integration with Git, SVN, and Mercurial.
Remote Deployment and Remote comparison.
PHPUnit Integration.
Portability (Windows, Mac OS, and Linux) and many more useful features.

PHPStorm and Netbeans are popular but the new Magento extension like Eclipse and Zend Studio introduced recently are also nice and getting more familiarity and popularity now.

The Magento Eclipse Plugin provides support of Magento projects for Eclipse-based IDE’s: handle Magento instantiate of models, singletons, and others by adding code completion to these methods; wizards that help to create initial module structure or extend native Magento module. This plugin extends original Eclipse/PDT one.