I love it. I played the single player campaign a lot in the original when I was younger but didn't venture into multiplayer. But I do enjoy multiplayer (well except 1v1s) in SC2. I've been playing since launch and I'm not bored yet... which is strange b/c I normally get tired of games much, much faster than this.
Tried going back to play SC1 again (omg, the music was FANTASTIC! SC2 definitely loses on this account) but I'm spoiled by the interface improvements. Hotkeying buildings, the ability to queue (even though I never queue more than 1 thing and only briefly), ability to group more than 12 dudes, awesome. I also really like the new macro mechanics (inject, mule, chrono) and some of the new units/abilities are tons of fun (bliiiink).
Oh for anyone that enjoyed the SC1 missions but wants the SC2 interface, some people have remade the whole campaign. It's actually a bit harder b/c the SC2 AI is a lot better than SC1 (e.g., zerglings are pretty rape b/c their swarm mechanics are much improved). I can't recall if they brought the music over or not. Also the 'dungeon crawling' missions are kind of lame b/c they do it in the 3rd person perspective which I find awkward as hell.