Percentage movement speed stacks additively with other sources of percentage movement speed.
Mercurial Scimitar works best as a late game upgrade for AD carries from Quicksilver Sash.
Mercurial Scimitar is a great escaping tool, but in order to utilize the movement buff and the removal of crowd control, the active is best combined with blink abilities such as Flash.
Mercurial Scimitar is the most defensive item choice out of all the possible upgrades for B. F. Sword.
If Quicksilver Sash is on cooldown and you upgrade it into Mercurial Scimitar, you recover the ability to use the active again.
Unique Active – Quicksilver: Removes all debuffs from your champion. If your champion is Melee, this item also grants 50% bonus Movement Speed for 1 second - 90 second cooldown.
Mercurial Scimitar is 99.5% gold efficient.
Quicksilver Sash.
Flash.
B. F. Sword.
Cleanse.
Infinity Edge and 
Lord Van Damm's Pillager for the most expensive item in the game.
