One thing I should point out is that the multiplayer ladder is very competetive. There are 6 main leagues and you'll struggle to get beyond the 2nd one unless you put a reasonable effort into learning the game (by which I mean playing matches, watching a few youtube vids and learning some build orders and basic tactics).
If you just want a casual game to mess around with an hour or two a week, SC2 is still a great buy, just don't bother with the ladder. Do the campaign and then try out joining some custom games. There are thousands of mainly player-submitted game modes some of which are really fun (tower defence, arena style, hero quests etc etc). You could literally never buy another game other than SC2, there is that large a variety of fun modes to choose from.
IMO though, the multiplayer ladder is the best part of the game if you enjoy improving and learning and honing your skills. It's great fun being beaten by a strategy, looking up how to counter it and then raping it silly next time you come up against it. The matchmaking system is awesome too, always finds players which are of similar skill to give you a reasonable challenge. There are a few annoying quirks (eg no way to tell how close you are to promotion, portrait farmers etc) but generally it works great, better than any other RTS ladder I've used.