Burst of Speed (E): -1 second cooldown, +15% AS, +2/4/6% movespeed.
How much is an extra 15% AS worth? In a duration of three seconds, not much. To save you the math, it buys you one extra basic attack about 1/3 of the time, depending on your level and items. That is roughly equivalent to a damaging spell that increases in base damage by 1/3*(total AD + on-hit effects).
At Level 3? That's 64.1/3 = +21.36~ damage per point, up to 81.01/3 = +27.016~ damage per point with AD runes plus a Doran's Blade or Long Sword. Compared to Lunge, that's half the damage per skill point and it requires a solid three seconds of autoattacking, rather than instant damage! CC will screw you: Irelia stun & run, Udyr stun & run, Lee Sin Safeguarding away, Teemo blind... Compared to Riposte, the four extra autos you get during a BoS is worth 20 additional damage just from its passive bonus AD, let alone the additional benefits Riposte provides. Oh, and the cooldown reduction per level is the worst of all her skills. It has the lower -1s reduction of Riposte but the high base cooldown of Lunge, so leveling it provides the lowest increase in uptime per point.
Burst of Speed gets better later on in the game, as autoattacks scale much better than Lunge and Riposte do. Maybe you level it second, so you'll have it maxed by the time you get hold of Wriggle's + IE and laning starts to break down, but the damage increase on Lunge will still be competitive then and the cooldown reduction on it is more meaningful.
except, at rank 5 it's 120% AS and 15% MS per stack