I jailbroke my iPhone 4 with the RC version of limera1n. Greenpois0n had not been released when I jailbroke 4.1. I had zero problems. It was an extremely easy jailbreak. Almost as easy as jailbreakme.com. The only disadvantage is the Windows only jailbreak - at least for now. Limera1n does not install Cydia by default. Click the limera1n icon and the option to install Cydia is there. I have not tried installing greenpois0n, but I would imagine that it automatically installs Cydia. As previously pointed out, they both use the same exploit and will suffer the same fate with the next update. Installing Cydia and the apps that can be found on Cydia is the major reason for jailbreaking (fun of doing it is a close second). limera1n and greenpois0n both accomplish this goal.