Is their any links to stock firmwaresI'm on O2 branded phone and might take the plunge
sammobile.com has them. It took about an hour to download the firmware as it used hotfile.com which deliberately throttles speeds for non-payers (can't say I blame them!)
After that it was completely painless, had the new firmware on within ten minutes, didn't delete anything (I did a backup of the internal memory first just in case). Haven't found any problems yet.
Another change I haven't seen mentioned is that the Samsung ChatON messaging service is included by default.
All of my widgets and home screens stayed the same, and as far as the device is concerned my phone is still official, so should be fine for warranty purposes.
Just read a guide before you flash, because if you do the wrong thing in ODIN you can brick your phone (usually fixable but takes time and effort).
Edit:
http://forum.xda-developers.com/showthread.php?t=1646610 - this thread has all you need.