The campaign game is quite well-designed. The scenarios introduce you to each of the three races, providing access to more and more units as you progress through the game. This is an excellent way to teach the player about the strengths and weaknesses of each race. The challenge also increases as the player progress through the campaign, providing good challenge in the latter stages. I recommend that new players play through the campaign before venturing into the online arena - it will prepare them for the challenges of facing human opponents quite nicely.
StarCraft is one of those rare games that is not overly complex, but provides a challenging and fun gaming experience. It never seems to get old or boring, and always provides a changing game experience. I'd rate it a 1001/10. StarCraft remastered ain't bad either. However, the original always rocks.