This is an ap supp item?

: Only good on champions with shields and heals : More of a QSS for Ap Supports : Passive Gold and Ap : Only good if your adc is good

Mejais is only good on supports if your team is getting decently ahead. You take it if you know that you are going to be getting a lot of assists without the need to have yourself die in teamfights. In this way, it stacks fairly fast and allows your support to have higher values on shields and heals due to its large amount of ability power. This is why it is good on Janna because she rarely has to put herself into a bad position to get a lot of assists, and the extra ap on her shield is extremely powerful.

It's the single most cost effective AP item by far; but only if you keep the stacks on it. If you don't need flat AP, or you can't get and maintain stacks (you either die too easily and/or you aren't getting much kill participation) then it isn't for you. It might be a good power boost with 10+ stacks mid game, but late game the other items are more reliable albeit less AP potential.

Someone explained why you see Janna's take it sometimes already. Rather than just the champion you're using, you need to consider the enemy as well; if they have someone that can get past your frontline and destroy you and escape safely (Zed, Leblanc) or flanks and takes out the back line that way (Twitch, Hecarim) then your stacks probably aren't very safe, especially if they decide to make your purchase a waste of gold by killing you for that reason.

If you don't have a good record of not dying or expect to be the focus of an assassin, it's best to just ignore Mejai's and (the AD version) Sword of the Occult's existence.