Air is best at mainly harassing, but they can of course also be a devastating force especially if countering an opponent with not so much anti-air. If your opponent scouts you going air though it isn't too difficult to counter it. Although Protoss has the hardest time countering air units. Generally you probably want at least some ground support, going only air is only good as a situational strategy, like if your opponent masses lings and roaches or something like that. It all depends, but yeah, generally you don't want to "plan" to go straight to air from the get go. You typically never want to skip tiers of units in favor of teching because if your opponent scouts you teching you're going to get caught with your pants down.
Again, there are always exceptions, but I think to be safe this is okay advice...feel free to correct me people.