Starcraft 2 has great campaing, but buying it for the campaign is like buying an adult movie for the plot. Its E-SPORT!
You can play the campaign and enjoy it. You will undoubtedly have great time playing through probably the best RTS campaign ever made. But once you finish it, it would be huge mistake to just uninstall the game and move on, because you have just barely scratched the surface of the fun that this game has to offer.
Instead go on ladder and play those five placement matches. It didnt go well? You had your ass kicked badly despite completing the campaign on hard difficulty? You are placed somewhere in the bottom of the bronze league? Good! Thats what happened to most of us.
Its time to learn the basics. Do some research, watch your replays and think about where you did mistakes, read through teamliquid forums (where most of starcraft progaming community dwells), pick one strategy and learn to do it right. Trust me that once you learn the basic mechanics, you will get into a higher leagues in no time. The real fun starts somewhere around platinum where both players know how to play through the early game correctly and matches get much larger and longer.
Think about it as chess - after you finish the campaign, all you know is how the pieces move. But that doesnt mean you know how to actually play chess.
Once you get good (or at least decent) at Starcraft 2, you will enjoy probably the single most rewarding experience videogaming has to offer. And once you decide to quit playing it (because from my experience you have to play at least ten hours a week to keep yourself in a good shape, otherwise you start drowning down on the ladder), you can still enjoy watching pro-gamers matches just like some people enjoy watching ice hockey. For people that reached certain level of skill in Starcraft 2 and got decent understanding of the game, watching progamers doing their equilibristics at 300 apm is insanely entertaining experience.
If you want to have some basic taste of what Starcraft 2 progaming looks like, check this video: http://blip.tv/day9tv/dreamhack-summer-2011-grand-finals-game-5-5296508 (its fifth game of Dreamhack summer 2011 finals between HuK and Moon).
note: This review was written by ex-diamond league Starcraft 2 player on European ladder.