Last week I purchased a new Samsung Galaxy S II. After a week of usage of the stock ROM and root-less capabilities, I was finally ready to root my device. The process for the Galaxy S II is pretty straightforward still. Unfortunately this install requires a Windows machine. At the same time, it has been the most error-free install I've seen to date. The process involves flashing the new recovery using Odin, then using ClockworkMod to install SU.
Most of the instructions from http://forum.xda-developers.com/showthread.php?t=1311194 work great. All of the files that you will need are there. The only additional step that I had to take was after rebooting my phone with the USB cord plugged in, the Phone asked me if I was willing to install custom software. I had to press volume up to agree to install custom software. After that, Odin proceeded perfectly. I was then able to boot into ClockworkMod, take a Nandroid backup (After I installed an required SD card), and install the SU zip file.