I'd get it, if I were you. It's such a good series! And I think you'll become addicted to the multiplayer pretty fast I've recently started dipping into old series I've never played before, i.e. BioShock & Dead Space, and honestly, there are no cons to trying a new game.
The first time you experience a series is gold, you should definitely get this game. Also, look into the renting the original trilogy. You can play the first 3 campaigns to get caught up story-wise, and you can get a taste of Halo 3 multiplayer as well.
Edit: Also Reach, don't know why I forgot to mention that one >_< While it isn't really essential to the main storyline, it contains a fantastic campaign.
Isn't this game $40 at a lot of places anyway? No reason you shouldn't go for it.
Looking for ODST players---PM me ~~Keep Calm and Chive On~~