The new break allows access to Cydia’s unofficial app store, the ability to install alternate settings and use different themes. The patch is 5MB and should be working for Windows and Linux in addition to the Mac version soon.
Nevertheless, there’s much speculation on whether it’s worth the download. Rumors have that Apple’s iOS 4.3 will be out next week, which could include the ability to use your iPhone as a Wi-Fi hotspot (on Verizon’s CDMA iPhone), not to mention that the iPad 2 could very well peek its head out. This feature and news could be worth waiting on for a newer jailbreak.
This paired with the possibility of bricking your iDevice and the fact that Apple has already patched the hole exploited by GreenPois0n, makes it seem like a fruitless endeavor, unless you really love 4.2.1. that much, or you just like to utilize the freedoms offered by the U.S. Copyright Office.
In addition, users of ChronicDev’s patch have said the program has failed unpack aging itself on occasion. Considering all this, we’d recommend you hold off until either the bug fixes are worked out, or Apple unveils a new OS for jailbreakers to hack.