She's overrated by many otaku, but that seems to simply be a trait that otaku can't shake - they overrate their personal favourites. I know I tend to do that. In general she isn't, though. Aya Hirano is usually praised as much as she deserves.
She is one of my favourite seiyuu, and though I didn't know about her specifically before Haruhi, I did know her from Kiddy Grade and Eyeshield 21 before that.