I first decided to root it after an OTA update noticeably slowed down the phone. Any custom ROM will do a lot for performance on the phone. The best forum for the Optimus S is on Android Central. Gingerbreak is the current method to root the phone… I used an earlier more complicated method.
The directions for using Gingerbreak can be found at:
http://forums.androidcentral.com/lg-optimus-s-rooting-roms-hacks/79208-how-rooting-ls670zvd-gingerbreak-no-reverting-ota-works-too.html
I am using the latest Stable build of CM7 from Drew. He takes CM7 and builds it based on CM7 nighties with a few builds being marked as stable. He introduces his own hacks to CM7 to optimize it for the Optimus. I was running an earlier port of CM7 but love the optimizations and fixes that Drew has performed.
You can find the rom and directions for CM7 at:
http://forums.androidcentral.com/lg-optimus-s-rooting-roms-hacks/106518-drews-cm7-builds-stable-08-02-2011-experimental-08-03-2011-a.html
I wrote this post for all those asking me why I rooted my phone and how they could root their LG Optimus S.