I find myself getting more and better use from move speed quints then any other possible quint, in addition to Move Speed quints max health % upper quints are about tied for all game scaling and usefulness(especially when "beyond the late game" in those 2 hour matches). Move quints on many champions(janna and yii particuarly come to mind) enable them to completely forgo boots entirely the entire game(and in-fact buying boots would lower their move speed in certain 6 item builds..) which gives them a in game gold value of roughly 1750 or so....gold value wise move speed quints hold a gold value of 750-1250 on most champions(especially any champ with 330 or better move speed, or movespeed masteries or self boosting effects), while flat ap/ad quints hold a gold value of no more than 400gold. When you break down your quints into their contribution in game as a value of gold you see tremendous power in Move Speed % quints.
Infact it is almost a garenteed win in any mirror match where they are not running move speed quints in solo/duo/jungle lanes during lane phase, and late game while....a champion with 360 base move speed and movespeed quints on will still catch you, in general you will always be able to escape most slowing, snare, and skill shot champions.
It becomes really absurd on 6 phantom dancer(or other % movespeed items) Hecarim where you hit the movespeed cap very quickly to the point of glitching because you are moving faster than the game engine can handle(cough cough, homeguard 5 5-10% move speed boosting items using E in fountain....cough cough). once you are at 650 or more movespeed you will never be slowed enough except perhaps by thresh's ult which will cripple you more than it would be if you had no boots and items on.
All in all i'd say unless a game lasts longer than 3 hours, Movespeed quints will give any champion the best bang for their buck, particularly the slowest, and fastest ones(with the least bonus given to those in the middle ground of the speed)....They also cause some interesting effects in dominion and with homeguard