I first noticed Hirano Aya when she dropped her normal voice an octave and gave Lumiere in Kiddy Grade an unexpectedly mature voice. After that, I don't think I noticed her again until Suzumiya Haruhi no Yuuutsu. That's pretty much when everyone else noticed her, too.
Hirano Aya became so popular after voicing Haruhi that the considerable backlash and stalkerish scrutiny to which she's consistently subjected now was inevitable. Her popularity also led to a lot of miscast roles as she became the new hot seiyuu. She was seeming crammed into every show possible whether she was appropriate for the role or not. (See, for example, Ayu in Hyakko.)
But when she's right for it, Hirano Aya is dead on. In the first season of Queen's Blade, it appears they gave her a case of Red Bull energy drinks and just told her to ad lib all her lines as fast as she could. It was magic. Kismet, you could say. Likewise, she was perfectly cast as the Kira-fanboy flibbertigibbet Misa in Death Note. It's largely thanks to Hirano Aya that Misa is basically the only character I actually like in the entire show.