WOW...I began my Halo career in 2008 with Halo 3. Halo 3 has been my favorite since then. I then went back and played all the previous Halo games (including the Halo Wars series) and also read ALL of the Halo books. I was thoroughly excited for this game...but it still surpassed my expectations! FANTASTIC!!! Brought me back to the old Halo days.
Please take my recommendation though...take your time! DO SIDE QUESTS.
The campaign is long but if you thoroughly explore the world and do all the side quests you will not regret it.
(also play on heroic your first time ;P)