I like her, personally. Not exactly in a bad place either performance-wise - she enjoys higher win rates than well over half of the AD carries out there (including Ezreal and Caitlyn, the two most picked champs in the game).
The reason you had 20 something assists and few kills, Dark, is probably because you overused her Q and in doing so had no regard for your mana pool (Erick's got a valid point). That's countered by using your W skill instead to farm minion waves, and Spellshielding whatever you can find to Spellshield, as doing so restores a lot of mana.
That and I believe it counts as an assist if an ally kills someone while buffed by your ultimate.
EDIT:
^this, she can easily wreck ADCs that use more spells than others and she can wreck Ezreal, Graves, and all those because of it. As for going against her, I really don't have a problem beating Sivir with more AA-reliant carries like Ashe and Twitch. When i'm playing as other carries like Ezreal, I may build an early Spirit of the Elder Lizard to deal extra damage around her Spellshield.