They are notorious for low pay, and you will be working alongside people from Accenture who earn ~30% more. Basically ATS was created to cheaply bodyshop people and somehow keep Accenture competitive with the other big cos.
I'm here. In case you're not aware, I work for Accenture Technology Solutions in Finland. Now, I'm really not confident about saying anything about ATS in countries outside of scandinavia but I've been more than satisfied during the past 2 and a half years I've worked for ATS Finland.