I'm pretty casual about LoL, but Sivir is my most played hero so I thought I'd drop my two cents. I just thought I would toss out my item choices for folks to look at, as they seems slightly different then what's listed here. Anyway, here it is:
Upgrade to Emblem, get Boots
From there I finish starks or banshee veil, depending on game, last item I usually go for is Infinite Edge, though other items can be used for specific situations (ie Wit's End against casters). I find rushing Cata gives me a ton of staying power in the lane, plus lets me jungle for buffs a couple levels faster.
Other then the item differences, I play her the same as everyone else, tear down creep waves and towers, support team with Ult, profit.
I'm unable to rate the guide for one reason or another, so I'll just say again, I really liked it.