The founder of Cydia, Jay Freeman, has announced that 4 million unique users have installed Cydia using the Evasi0n jailbreak. That jailbreak became available on Monday night. Six hours after the jailbreak became available it was already downloaded more than 800,000 times. On Tuesday the download counter was at 1.7 million downloads.
This makes the Evasi0n the fastest spreading jailbreak ever, probably because it took some time before it was released. Besides that, it's the only available jailbreak method for iOS 6 which means that owners of an iPhone 5 or iPad mini can only use Evasi0n to get full access to their device. Another factor that could explain the popularity of the hack is that there are more owners of Apple devices since previous jailbreaks.
The jailbreak can be used for devices running iOS 6 and iOS 6.1 and makes it possible to use tweaks that are prohibited by Apple. The jailbreak can be applied by downloading an application for the Mac or the PC. The jailbreak also makes it possible to install applications without using the Apple Appstore, something Apple also doesn't allow.