Some people take mejais all the time regardles of what team they're facing, this is most of the times a horrible mistake. Mejai is an item you take after your first bluepill and you have noticed that the enemy team is either really bad or has a weak link (IE someone you can kill reliably). THEN you get a mejai's and capitalize on those flaws for your first stacks, then as you get progressively stronger you can start to pubstomp people.
An example of a bad use of mejai's is the guy on mid lane taking it as his first item against whoever he's fighting and end up losing mid or not getting any kills at all because he spent money rushing mejai's while the other champion spent money increasing his lane-staying power and farming. I find that 70% of the time, its survivability and not damage potential that defines who is going to win mid lane. Of course skill is aways an important factor, but not as much if you really can't kill the enemy mid champion by any means.
This is what makes cho a god tier solo champion, his lane-staying power is far greater then all of the common mid lane champions, even after his nerf. Even AP cho has much more survivability then, say, tristana, ashe, ezreal. Chances are he wont carry your team, but he'll cripple the enemy carry and win you a lane unless he's ganked several times or being terribly played.