This bit is the problem. I don't jailbreak a device before I am ready to throw it away. I've seen too many issues with non-official hacks of various sorts. Nothing major; just rough edges.
They even provide an official tool for rooting.
There's pretty much no chance of bricking your phone, and if you go with a stable CM build, you shouldn't have any problems.
http://get.cm/?device=i9300&type=stable
... while the Galaxy Nexus has already gotten its OTA update. Closest I've found is the SuperNexus ROM, but even that's still glitchy.