No, they aren't, not directly anyway. It's hard for attacking players to get enough space to get an open chance, even when they do get a good chance, there is usually a defender very close. One of Maguire's headers last week everyone was giving out to him for missing but on replay you can see the defender actually got a touch to the ball before him. Most of the time the ball is swung in with so much power it's impossible for an attacker to get a meaningful header without it going wrong. The second ball, a knock down or flick on, is probably a better opportunity. They introduce panic and disorganisation in the defence.
That's in addition to the fact that taking the perfect corner is really fecking hard. You need to send it in with enough speed that the defence can't easily adjust and clear it, which in turn makes it harder for an attacker to hit it on target. You need to hit it high enough to beat the initial defenders, but low enough so that it doesn't sail over everyone. Despite everyone hating them, you really are better off taking a short corner and hoping the defensive structure is broken and one of your attackers can get away from the defence and person marking him.