Honestly, you've never seen a good Sivir. Her early game damage is ridiculous, she dominates bot lane against nearly every other bot lane champion and late-game she becomes a hyper-carry. She has a 15 second AoE movespeed buff that also affects minions, and can block one spell every 10 seconds.
She's beastly on CS.
Your assessment is accurate except that the problem with sivir, even a good sivir, is that she is hard countered by Thornmail. I say this as someone who has been wtf-pwned by sivir multiple times. Every game it turns around the second I get thornmail. Considering that 2k is rediculously easy to get in Dominion...I back up his low assessment of Sivir. It's a shame but there you go.