I'm gonna go out on a sorta TL;DR thing, so bear with me.
Halo 3 was brilliant. It's my second favorite in the series with Halo 1 leading it and Halo 2 and Halo Wars trailing right behind Halo 3. That's not to say that they were bad, one felt like it was running out of steam far too quickly and the other felt unnecessary, albeit fun. So what happened with ODST?
Well, one reason I buy and play Halo is because of the Campaign and story. It may make no sense to some of you, but those who stuck with the series since Halo Combat Evolved would understand what I am talking about. The story for Halo 3 was there. The campaign? Not really. Oh sure it was a little better than Halo 2's campaign, which felt in stark contrast like one massive corridor shooter, but really, it didn't feel like it was an improvement in anyway. It was more a step backwards, which actually felt heartbreaking when you thought of the piss poor length of the campaign.
ODST feels like a breath of fresh air because of how the game paces itself. Slow, yet tense, with spurts of adrenaline. It was a much more different approach to the Bungie Manifesto of Thirty Seconds of Fun. (IE: 30 seconds of fun repeated over and over again.) BUT it's not without issues. The SilencedSMG was a great replacement to the BR in many respects, while the M6S, the suppressed version of the Halo CE M6D felt like a weak and puny peashooter. Worked well on the head, but sucked when shooting anywhere else on your target.
As for the storyline, there really wasn't much of one. It was just being unraveled as the campaign continued towards the end. But it was how the storytelling was done that set the game apart from how storytelling in Halo is handled. Telling the story via the flashbacks and Sadie's Notes was a very interesting concept because the real mystery wasn't in what happened to your ODST buddies, but with the city as a whole. Seeing as the game takes place in between the events of Halo 2 and Halo 3, this would've been an excellent opportunity to do the same Halo 2-esque perspective switching between well... Arby and the Chief without it feeling as jarring. They didn't. They missed a golden opportunity to characterize the enemy threat and understand the story from both sides like Halo 2 did remarkably well.
And then it falls apart. The ending missions to Halo 1, 2, and 3 were equally memorable and the best parts of the game due to how much more they pushed the boundary and made it feel like an epic finish of an epic journey. ODST's last mission was, needless to say, piss poor. The last mission was the same "Run with Warthog" premise as Halo 1, but aside from that, there was nothing past it. It was basically, "Hey! Drive around with a Warthog protecting Vergil from damage, run back to a base, play a hack version of Firefight IN THE FUCKING CAMPAIGN, run to the Phantom, DUN DUN DUN! You're done!" Yeah! That's it! That was the end of Halo 3: ODST. In spite of some of the best levels and set-pieces in the entire series, it just crumbled towards the end, and that really disappointed me a lot.
At 6 hours, it's hard to say that the game wasn't worth it. Yeah, I felt cheated out that they put all the DLC on a second disc, but then again, I already have Mythic 2 on DLC, so why bother with the Mythic Disk when Halo Wars packaged the first three maps on the Limited Edition? So was it worth full price? Well, the campaign really has its moments, sure, but not worth the full price tag. But you know what?
Playing the game right after the Halo Reach Beta launched, I learned something about the game I didn't know about. This isn't an expansion pack. It's a statement. This is going to be what we should expect from Halo Reach, but multiplied by a thousand. We've seen a lot of coverage of Reach and what will be in it, and it's obvious that Halo 3: ODST is really meant to be a taste of what Reach is going to be. Maybe not verbatim, but the health system, lack of dual wielding, the sense of vulnerability, and the much more "human" story Bungie wanted to tell are things that will be present in Halo Reach, but in greater detail and care. Everything from the campaign to firefight from the past games will be in Reach, but with much more polish and features.
So while I enjoyed Halo 3: ODST, was it worth it? Probably not. For a six hour game with an open world padding, and all the maps everyone paid plus the three Mythic 2 maps, it deserves a cheaper price. Thankfully it's already got a lot cheaper, so if you're curious, or a fan who couldn't afford the game when it came out, get it on the cheap.