To be honest, its up there with the likes of Op For and Blue Shift for expanding a franchise story properly (Pro-Half Life, Anti-Halo peeps please hear me out before flaming).
Having played it through once on Co-op Heroic and still looking for all the hidden stuff in single player Heroic, I can say that certain parts of it make no real sense when compared to the main Halo game, but taken as a stand alone product rather than an 'expansion pack' it is still very enjoyable and has a lot more focus on background storyline.
But then, perhaps, loving Firefly and Serenity, I'm just a Nathan Fillian mark. The squad dynamic is cliche enough when it needs to be, without grating enough all the time, and a lot more emphasis has been placed on body-language over words than has previously been seen in most games (not quite Heavy Rain or Dreamfall but a lot better than the likes of say, Jericho or even Gears 2)
Kit and abilities wise, no dual wielding, no 'x' button gadgets (tho the brutes use em all the time) and even on Heroic I found myself possibly taking more of a pounding than Master Chief ever did (Bugs are a nightmare of flying plasma madness). The silences pistol seems to do the proper job, though the suppressed SMG comes off a little underpowered at times (but the pistol is rubbish at popping shielded targets in comparison). Strangely, yes, you can still rip turrets off their mounts, something only Spartans used to be able to do, and it looks like you can certainly move a heck of a lot faster with them when you do, but then, this comes across more of a gameplay tweak. The larger covenant weapons, such as brute shots, fuel rod launchers and beam rifles pack a far more significant punch than they did against the Chief's shields, and vehicle weapons will rip you to pieces, so you do need to think a lot more about how you approach a situation (the overcharged plasma pistol tactic being even verbally suggested to you at one point by your squad mate, something a large amount of dabbling Halo 3 players probably won't have even noticed you could do). On a similar note, the AI is a LOT better, though sometimes doesn't listen to its own advice (Captain Dare orders you to take point, but moves faster than you can sometimes anyway), and the bits where you are driving Warthogs with them on the turret they are far better shots than you'll ever be at full speed, a massive departure from the previous troopers who would happily shoot the cover a grunt was hiding behind over the onrushing ghost.
As for the length issue, Halo was, is, a game than manages to bridge the casual/hardcore gap a lot of the time. Someone without much experience can enjoy a good game of co-op (online or off) and not feel too overwhelmed by it, and this also extends to multiplayer deathmatch etc. Locally we have LAN parties where we get a few mates over with their Xbox's and a few spare PC monitors and some beers and have a nice, fun, banter-filled session. Can't do that with CoD as you can only do one player per screen (and we aren't made of money to THAT degree), and the likes of Rainbow 6, and the upcoming Operation Flashpoint are way to lethal for someone with little knowledge of the game system and maps to even score one kill in without a measure of luck. Playing Co-op is fun even when the game has been completed over and over, and anyone who complains the single player campaign is too short obviously isn't a person who actively looks to find everything and will never be happy with it.
As for the price, shop around. Locally I picked mine up for a just a few pence over £30, which is almost £15 less than the average current asking price for Modern Warfare 2. Firefight mode is fun, if not exactly original, and the Reach Beta gives a reason to hold onto the disk over trading it in.
I'm not a fanboy, unless I am by extension a fanboy of every game I own, I just know when I like a game and know when I want to own it. Despite some people's mistaken belief, nobody 'HAS' to own anything, it is their own choice and desire to do so in the first place. Did I have to buy ODST cause it was a Halo game? No, certainly not, and I decided to only about an hour before store closing time, so its not like I rushed out to get it. Incidentally, the Tuesday release did kind of help... as it means the game is now played and finished and I'll have a lot more time with Scribblenauts on Friday!
In summation: Is it worth it? Yes. Is it good? Yes. Is it short? Maybe, but then its not an RPG, so I'm not expecting its solo player to last me weeks.
