It's been a week since release, and I've pretty much played nothing but ODST. If you're on the fence about the game, maybe my impressions will help you decide if it's worth it! Here we go:
Campaign
The campaign is the main reason why I bought this game. For some reason, I think I enjoy playing Campaign more than I do playing online in matchmaking. Plus, what's more fun than 4-player co-op over Live? Anyway, back to ODST. I absolutely love the main missions. Each character has a "specialty" as an ODST (ex: Romeo is a sniper), and although you can use whatever weapons you want, you'll find that each level is basically designed for that character's specialty. There's a sniper, explosives expert, a guy who loves big guns, etc. You probably won't be using the same old BR/Shotty combo in every level like you could in Halo 3. Well...you definitely won't be using the BR/Shotty combo, since BR's aren't even in this game!
It's been said many times before, but playing as an ODST is very different from playing as Master Chief. You'll feel VERY vulnerable to everything around you. It's rare that you'll go through a stage (let alone a single minute) without some sort of frantic, high-tension, "holyshitomgi'mgoingtodie" situation. Also, ODST looks noticeably better than Halo 3...but the engine does sort-of show it's age. It's no Forza 3, but it still looks pretty good.
The story is decent; it at least made more sense than Halo 3's story did. There's still some pretty big holes that you'll have to read the books to fill. As always with Bungie, the dialog is sometimes mediocre but overall pretty good. My favorite line goes something along the lines of: "That... was a lot of people dying at once!" Story progress by searching around New Mombasa as The Rookie, finding clues, then having "flashbacks" to find out why the clue is there. It's a very clever method of giving the would-be jumpy story a strong sense of continuity. It's a lot more interesting than just playing as the same character mission to mission like every other FPS since the conception of the genre.
I'm still not sure if I enjoy playing as The Rookie or not. Exploring the city is kind of fun, but there really isn't enough variety in the design of New Mombasa to make it as entertaining as it is probably meant to be. Granted, it's supposed to be in ruination and darkness, but nearly every area looks the same. Of course, you'll be using VISR mode to see things in the dark. Although it's really useful having enemies outlined, I'd rather just be able to see them WITHOUT VISR mode. There's a sort of collectible in the game in the form of audio recordings, but they're pretty hard to find in the dark environment. I did laps around the city last night for an hour or so and I'm still missing about 10 of them...I have no idea where they are, and not much motivation to go and find them. I'd tell you how interesting the audio recordings are, but I haven't paid much attention to them. Fortunately, you don't HAVE to play as The Rookie very much at all if you don't want to.
Firefight
I've only played this mode once this week, but it's really fun. I'd played on Easy with one friend just to get a feel for it, and it was exactly that: EASY. It doesn't take a whole lot of effort to just survive, which is actually a plus. You can plan on spending two hours or so on Firefight and know that those two hours will be used up, unless you do something really stupid. It's all just crazy-awesome fun, particularly the bonus rounds. So far I'd have to say Firefight > Nazi Zombies. I'd like to talk more about it but I don't think I'll have a very good grasp on the game type until I do some 4 player Legendary action.
Halo 3 Multiplayer
I only had the Heroic Map Pack before ODST, because I bought Halo 3 so late...so I've had a shitload of fun trying out all the new maps. My favorite is definitely Longshore, one of the 3 new ODST-exclusive maps. The environment is a cloudy, port-like area along the water. If you haven't seen screens, I'd go check some out. Plus, it's a really well designed map. Heretic is a fan favorite too, being a remake of that one Halo 2 map that I don't know about because I haven't played Halo 2. Citadel is pretty fun too, not a bad map, but not a particularly exciting one either. Unfortunately, none of the new maps seem that practical for Forge mode. Longshore has some opportunities, but if you're big into custom maps, you probably won't find anything special with these new ones. But for the majority of us that just want some slayer/assault/CTF action, I can all but guarantee you won't be disappointed with them.
So that's what I think about ODST. $60 well spent? Hell yeah!