I just finished the campaign, and I'm pretty unimpressed. The individual missions were ok, each one has it's own little gimmick so YMMV. I really liked the day/night cycle one. None of them really play like a RTS in a traditional sense (at least that's not how i played them). I definatly liked the out of combat adventure mode bits the most. They do add a lot to the atmosphere, but it gets really repetetive after a while. Generally towards the end everyone who can say something will say the same thing everyone else does.
The plot is flat out ridiculous. First of all, there is definatly a HUGE cliffhanger. When they announced the game was being split into three Blizzard assured fans taht there wouldn't be any cliffhangers between the games. It only really resolves one part of the narrative, with everything else either being introduced and forgotten, or just ignored entirely. It also has a really poorly done moral choice mechanic where you can choose between two missions a few times. Overall it adds nothing to the plot really, and just slightly changes some ingame bonuses (do you get a ghost with a snipe attack or a "spectre" with a mindblast attack instead)
There's also a point where Raynor decides to join forces with his arch nemesis' son for really no good reason at all, engendering the hatred of everyone around him, and acting like a complete tit because plot dictates it be so for the big badass speech he gives later on. This isn't really explained at all, and is a real head-scratcher. Why this wasn't a moral choice is beyond me. It's probobly the single most important decision in the game and you have no say in it.
I'd expect the next campaign to be the zerg, and to continue almost exactly where this one leaves off, because otherwise there are going to be MANY unanswered questions to explain.
It pulls a bit of a mass effect 2, in that you spend most of the game doing missions to unlock units, getting one per mission, untill you reach a point where you don't get anymore and it's the end battle. Nearly all the missions before the endgame bit can be beat by just building whatever that missions unit is and swarming around. If you're playing a mission taht gives you firebats, just build unlimited firebats and you'll win easily.
The terran "metagame" of upgrading your units is kinda cool. Though I very quickly became annoyed by it. See there's two ways of upgrading. Many units have 2 upgrades you buy for them, such as the battleships emergency shield and anti-air missle swarm, and you can also use protoss and zerg techpoints (which you get in missions) to research new tech. The problem I had was that "researching new tech" equates to picking from a set of two options at 5 or 6 milestones. It's really frustrating to get to the top of the tech tree and not be able to unlock any of the other ones (it lets you sell extra techpoints for money, but that's kinda meh). I guess if you really dig replaying the game you'd enjoy this, but as the missions are allready so gimmicky there's just no draw there for me.
Also as far as I can tell, while you can go back and replay missions you've allready beat, you can't do it with your current tech. So no going back and playing the day/night mission with ghosts. There might be a way to do it, but I couldn't find it.
Overall the singleplayer seems kinda unfinished. The plot definatly isn't, and lots of little touches in the rest of the game seem kidna half done. You can goto the armoury to see some of the units you've bought, and read a little piece of fluff about them... But only certain random ones. The game was apparently "too massive" to be shipped with the other two campaigns, yet it doesn't really convey that, at least not to me.
If it was a console game I'd say rent it, but as it stands I say borrow it from a friend or something. The plot is a forgettale mess, the missions are interesting but easy, and the upgrade system and "branching storyline" don't really add anything beyond some vestigial replayability for those that need to unlock everything.