Pros
ATC has good benefits: RSUs, ESPP, 401(k), surprisingly good insurance, and I personally was paid well enough. Additionally, if you learn how to navigate the system (at least in Engineering), keep your head down, and don't make waves, you can wile away your 8 hours a day doing next to nothing.
Cons
I found the culture at ATC - at least on the Engineering side where I worked - demoralizing and oppressive. Though there were some decent individuals, in general, the management was incompetent, hesitating to make simple process changes that would have made life easier for rank and file employees for no other reason than that they didn't want to stick their necks out. Their incompetence, of course, made them insecure, and their insecurity made them oppressive, so in stark contrast to their feckless dithering when it came to improving process, the one arena in which they could act decisively was when silencing criticism. And as if the intra-group oppressiveness weren't enough, there was inter-departmental antagonism and mistrust aplenty. I personally saw emails where people in the Finance department did little more than point fingers at the Operations and Development teams, and I was involved in Operations trouble-shooting sessions where the express goal of the meeting was not to find the root cause of an incident but rather to find a way to absolve Operations by pinning responsibility on the Development team. In an atmosphere like this, an employee generally has three options: 1. Stop caring, keep one's head down, and do as little as possible. 2. Work the system and become as slimy an operator as the managers. For example, get yourself peripherally but non-essentially attached to as many projects as possible, then walk nonchalantly away from those projects that are bound for failure while associating yourself more closely with those projects that seem headed for success. Never take responsibility; always take credit. 3. Leave. If you take a job in the Engineering department of ATC, you can reasonably expect most of your colleagues to fit into one of those three groups, so if you are currently entertaining an offer from ATC and you aren't a slimy political operator, then I would sincerely advise you, as someone with no stake in your decision, to reflect soberly on your options before accepting their offer.