I love running Movespeed Quints and want to tell you why I think that MS quints are the superior choice on alot of champions.
The most obvious answer is roaming. MS quints enable you to travel alot faster from one point to another, that includes going back to lane (losing less creeps after recalling) ganking sidelanes and securing/stealing objectives. At this point people will still argue that MSquints are alot weaker than ad/ap quints or whatever you would use instead.
Here is why I think differently: While laning, extra ms enables you to abuse your range advantage more easily/deny the range advantage your enemy has. You can kite and juggle with ranges alot easier. Ontop of that, it enables you to simply run away from the jungler with superior ms/catch laners while ganking. All this is assuming the other guy doesn’t have ms quints. Now imagine your enemy having Msquints while you dont have them. They will be able to chase you down in any given situation unless you blow a summoner or a utilityspell (which you may or may not land/have).
What happens in teamfights with extra movement? As an AD-carry you can land extra autohits without having to reposition as often since repositioning is more time-efficient now, effectively increasing your dps by more than those lousy adquints could give you (thats in most situations, but you get the point). The same concept applies to casters aswell but not to the same extent. If a bruiser has ms quints and sticks to a carry the carry will have significantly more trouble kiting (effectively decreasing the ad’s dps).
After illustrating these points, people will probably realize why I myself and many other players think that janna is op (she basically gives everyone two free ms quints).
I like to use msquints on almost every jungler (I rarely jungle tho), most toplaners, APs whenever i can manage the matchup without flat mr or i dont have ridiculous range where extra ms hardly benefits me and on a few ads (varus,vayne,ashe,kogmaw). All of that depends on the matchup tho since you will need to evaulate every single game weather you can make it without the early powerboost ad quints/ls quints/… give you.
If you still dont believe me, give them a try and judge for yourself.
Please comment if you disagree or if I didn’t state my points in a comprehensible way(sometimes I get to caught up in what im trying to say to actually say it).