Starcraft 2 is a great game in terms of balance, learning curve and depth. Obviously since Starcraft 2 has gone through about 2 years of beta testing the balance feels just about perfect. The best I've ever had in a RTS game. The learning curve is great, with a great campaign and tons of single and multiplayer aspects to teach you how to play at a high level quickly.
The campaign is great; the choices you make affect the outcome of various characters, there is an ability to buy upgrades for your favourite units and you can unlock achievements, decals and portraits based on the difficulty you play at and the bonus objectives you complete.
Online are the best parts of the game (in my opinion): Custom games allow you to play games made by other people online in unranked battles, leagues measure your performance against other players in ranked matches or you can play cooperative group games against a.i.
In summary the campaign makes single player a great experience and the multiplayer online is what will keep you playing the game in a years time. If you are familiar with Warcraft 3 you know that custom games will be a great feature once modders have had more time with the game.
Buy This Game! You won't regret it!