I feel like magic penetration does nothing. I know that ap champs aren't support to destroy tanks but im fed most of the time and I feel like my magic penetration should be noticeable. Sometimes I build 
and 
and it still does nothing and that's like, about all the magic penetration items there is.
it a nitch stat needed for fighting tanks, as most champ have low base mr, and most dont have mr per lvl you wont notice the effect as much when fight carries
Penetration items like and 
(later as ) are good against low MR targets.
only reduces MR if your within a certain range.
And finally is at its best when the enemies have lots of MR. I hope this helps is some way.
I've been able to get 70 magic pen, enough to get true damage on low MR targets, and I have to say
it is MESSED UP on these characters.
Amumu brings it to 95 and Fiddlesticks brings it to 80
