Dude it's totally worth it.  I still play Halo 1 on occasion.  Story wise, the first one is probably the most enticing.  2 is pretty good as well.  3's probably the worst one, but still pretty good.  You might as well.  They're all great games.

I got a hold of Halo 1 a few months back, thinking I'd just play it for a bit and lose interest as most old games I dig up usually do for me, but to my surprise it actually lasted and I completed a good part of the game before putting it down (to play Mass Effect 2 which came out then) and I remember it being a lot of fun, it felt right (like all the Halo's did to me so far) and the combat was very engaging and challenging.
A severe change from the Call of Duty type games where you just mow down rows of generic enemies with generic weapons.
 
For an old game, it plays pretty new.

The original Halo has aged really well. Halo 2 has a few visual issues when you're playing it on a 360, but it's highly playable as well, and in fact, might even feel as tight as most of the newer games, since Halo 2 is where dual-wielding and vehicle-hijacking were both introduced.  
 
Definitely go back through the series. Totally worth it.