The campaign in all the major Halo games is definitely worth it from a story-telling standpoint. Unfortunately, the chronology is all kinds of messed up, especially from a technological standpoint.
Since Reach is technically the first game chronologically, you go from having awesome advanced armor kits and fancy and varied Covenant weapons and DMR rifles and Spartan Lasers to suddenly no Spartan Lasers, no BRs or DMRs, no SMGs, only Plasma Rifle/Pistol, Needler, Assault Rifle, (sniper) Pistol, Sniper Rifle, Shotgun, Rocket Launcher, then in Halo 2, no more Assault Rifle, no more sniper Pistol, we get SMGs and BRs, can dual-wield Needlers, Covenant Combines, Brute Shots, Brute Spiker thingies, can finally use Fuel Rod Cannons and Swords, and then we hit Halo 3, which brings BACK the Assault Rifle, can no longer dual-wield Needlers for some reason again, and then the Spartan Laser comes along, and all of a sudden we get deployable equipment like Grav Lifts, Bubble Shields, Power Drains, Flares, Radar Jammers, and Landmines.
While in Reach we had Jetpacks and Armor Locks
and the ability to run fast which was taken away from us
Oh well! Like I said, the story is still hella good. Best played in chronological order for a storytelling experience, but from a gameplay perspective, do it in the order released.
Note that I haven't played ODST so I can't comment on where that stands within the chronology or technology issues.