I have live and i play it every day just because i love to virtually kill people :P
I only need to beat 2 levels to complete it on Legendary(i've beat it once with my friends Xbox360 when he got it before i did...like 4-5months ago.) i just use mine for LIVE.!(epicold...hinthint)
The game...like previous ones, has A LOT of secrets and easter eggs., like CUTSOM armor.(which is a big plus.)
Heres a link,
Halo 3 Guide - Multiplayer Strategy, Maps, Weapons, Vehicles, Tips and Tricks
It provides all info and little tidbits that may make or break you as an online player.