I rooted my Nexus S 4G to play with all the custom ROMs. For many phones- from what I understand- there are one click roots. I know there's one for the Nexus S 4G. With something like that, the chance of screwing up is slim. Even if that's not an option, as long as you follow the steps from one of the many guides posted online, you shouldn't have any trouble. I had one little issue when I tried to flash a non-ICS kernel to ICS, but other than that, it's been problem free. Though CM7 was pretty buggy for the Nexus S. Lots of battery pulling and some weird screen issues. And ICS is murdering my battery life, though that'll probably be patched up soon with better ROMs and kernels.