Having played every Halo game except ODST
Buy ODST and play it. It's more enjoyable than Halo 4, easily.
Halo 4's campaign is very pretty to look at, but it comes at a pretty big cost. Everything is far more linear than any other Halo campaign, and the new Promethean enemies lack proper damage feedback, have an absurdly dull combat loop (Watchers --> Crawlers --> Knights), and are incredibly frustrating to fight at higher difficulties.
The sound design is spotty; the Warthog is so loud that it overshadows dialog at times, approaching Ghosts often don't make any sound, Covenant weapon sounds were needlessly changed and opinions on them are mixed. Then there's the soundtrack, which is generally pretty low in the mix and cannot be changed, which I feel does Davidge some fair amount of injustice.
The story is...meh. The Chief/Cortana plot is the most entertaining plot point but ventures a little to far towards actual romance than was probably intended. The main antagonist is your typical one-note BIG BAD, and it's an utter shame that the essential backstory needed to make him more intriguing requires you to physically stop playing the game and visit the Waypoint app to even watch it (can you even do that without Live? I dunno). I will say that the in-game cutscenes are incredibly well done and has some of the finest voice acting to date of the Halo franchise.
If you can find Halo 4 for dirt cheap, I'd say go for it. Amazon has it new for $21.99 in the US, and I'd say that's a toss-up on whether the campaign is worth it.
And be aware that, along with multiplayer, Spartan Ops (343's attempt at making their version of a Firefight/Campaign hybrid) is unavailable to people who don't have a Live subscription.
