I enjoyed it. Nothing mind-blowing, but I've seen a lot worse.
I never quite made the 'connection' with the original SC though. I mean SC2 is a "sequel" but it really - to me - felt more like its own alternate reality / spin off variation. And to be honest it probably was, it's just that they labeled it with a "2" for obvious advertising and hype goals. At least it's my perception of it. It really didn't connect well with SC1, in my opinion. However, with this said, on its own I still enjoyed SC2 enough. It's a bit like Mass Effect 1 Vs 2, for me (whereas I never quite considered ME2 a canon follow up to ME1, but in its own right I still liked ME2 for a couple of reasons).
I only played SC2 for the three campaigns though, never really touched the multi-player (same with SC1 for me). So after I was done with the last campaign I played some Versus A.I. skirmishes for a day or two and I was done. I haven't really got back to it. When it comes to RTS gaming, and fun (from the genre) I actually prefer the likes of Warhammer 40K, or classics like SC1 (if I had to choose, for the gameplay that is) or Rise of Nations, Command & Conquer RA2, Age of Empires 2 or Star Wars Galactic Battlegrounds, and so on. I think that SC2 was enjoyable for the cinematic presentation aspect of its campaigns. It probably was the major attractive 'branch' of the game for me. You "play a small movie" under the RTS view, you have some genuine interesting gameplay builds to try out (upgrading units, giving them specific types of attacks or defenses and so on) and that's about it. It's really a rather straightforward, simple enough experience overall (the campaign I mean, I know the multi-player part is a lot more complex than this).
I think I enjoyed all three campaigns almost equally. But contrarily to some here I actually think it simply got better with each campaigns. The Terran campaign felt very generic overall, with maybe the exception of the last few missions when finally the stakes are raised much higher and things are taken to a more epic scale. The Zerg campaign finally gave me the better close-up on the Zergs I had been wanting to see all those years about the StarCraft universe, it may not have been the "best" way, but really I was ready to take it, and I honestly enjoyed it a lot (especially Abathur, he's amazing, he's the "show stealer" of the Zerg campaign, and Zagara ain't bad at all; Kerrigan is 'ok', nothing crazy but I liked her role in it nonetheless). The Protoss campaign was the full blown Sci-Fi aspect of SC kicking in, in my view. There's (small) parts of it that kinda irked me (story, writing-wise) but overall I really enjoyed it. I can't really say anything about the ending to avoid spoilers, but I will say that I liked most (although not all) character progression developments at that point in the campaign (I.E. considering where those characters 'started' from, and where they ended up by that point; their growth). There's only maybe 2 of them in the end that I was like... "R- really...? Wow, ok." But that's about it.
I'd give SC2 (for the campaign at least, and the gameplay within the campaign, excluding the multi-player) a solid 7/10.