ARHD - Android Revolution HD
http://rootzwiki.com/topic/10518-rom-andro...-ics-legendary/
- based on GNex factory image stock rom
- no visual tweaks & enhancement
- applied performance & batt life tweaks
- rom optimized to EXT4 partitioning as per his Superwipe Lite
- minor updates for bugfixes and newer version of stock apps
- major updates only after 1-2 days Google do an official update on our GNex
- if you prefer fast/stable/like stock rom.. go for this custom rom
- average battery life (compare to stock)

AOKP - Android Open Kang Project
http://rootzwiki.com/topic/11455-rom-andro...ro-milestone-2/
- based on Google AOSP version 4.0.3
- lots of additional features (nav bar quick settings/fix auto brightness/softkey mod/1% batt mod/color mod)
- buttery smooth transition and animation
- frequent updates of bugfix and new mods
- a bit above average battery life (compare to stock)

GummyNex
http://rootzwiki.com/topic/13406-romaosp40...nex-050-011012/
- based on Google AOSP version 4.0.3
- lots of additional features (refer his thread.. long time have not use this rom)
- comes with custom kernel for better battery life
- periodical update of bugfix and new mods
- better battery life.. average smoothness
- HTC like selection sound... not like our stock tek tek tek sound

Bugless Beast
http://rootzwiki.com/topic/12874-romaospan...-bugless-beast/
- based on Google AOSP version 4.0.3
- lots of additional features (refer his thread.. long time have not use this rom)
- periodical update of bugfix and new mods
- need to refer to his download page to check for latest updates

if you really want more performance or more battery life
you can go 3 ways

1st way (not advised for beginner or intermediate android addict)
install setCPU and tweak your CPU speed
undervolt each CPU frequency u set for better battery
expect Sleep of Death or random reboots if your setting is not nice

2nd way (not advised for intermediate android addict)
flash a custom kernel after you flash your rom according to you android version
install setCPU and tweak your CPU speed
undervolt each CPU frequency u set for better battery
expect Sleep of Death or random reboots if your setting is not nice

3rd way (safe enough for beginners)
flash a custom kernel after you flash your rom
no need setCPU.. just use your phone as normal
expect some lags especially after unlocking your screen
you can ask to their forum topic if you encounter any issues
hardly you will encounter sleep of death or random reboot
confirm better battery life

so.. for those who are new to flashing.. try flashing roms 1st... try out a day or two.. after u found your best rom.. then you can venture to custom kernels

to answer what is the best rom n what is the best kernel...
it super subjective..

i have found my fav rom..
and since last week till now.. im seeking my best kernel...

currently using AOKP milestone 2 + testing Apex V5 kernel

sorry for the long post

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Added on January 12, 2012, 6:40 pm
ROOT allows you to access system files. With this access you can install certain apps that need access to system files , eg Titanium Backup ( a restore-backup solution for apps n data),  ADfree - eliminate those popping up ads, setcpu ( to set CPU speeds and voltage controls ), etc etc. It also will allow you to install custom roms which are generally more efficient than stock roms

You can refer to SGS II official thread post #5 where I had compile a short guide about it:
http://forum.lowyat.net/topic/2178338

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Added on January 12, 2012, 6:43 pm
RIGHT ON ! If you are happy with the phone performance it is not really necessary
It is for those who want to squeeze more mileage out from their car ,