Pros
They don't hesitate to throw 'goodies' at you, like free food (tons of it), company lunches, happy hours, promotion parties, etc. There are some truly passionate people at Digitas and if you're lucky you'll get to work with them and be inspired. The Mentor/Mentee program is a great way for junior employees to meet and talk with someone more seasoned in their field and get some great advice from them about the business as well as their personal career aspirations. It is also a good name to have on a resume if you're planning on a fruitful career in the ad world.
Cons
They work you like dogs. In fact, it seemed to be the sentiment that the longer hours you worked, the more glory you had as a dedicated Digitas employee. Almost everyone in the company is overworked, and senior management has pretty much no work/life balance, as it didn't seem to matter whether or not they were on vacation - you could almost always count on getting an email from them if they needed something from you. (But if you're waiting for an important reply and you're not on their priority list, forget it.) Also, if you weren't a billable employee (in other words, if you didn't directly contribute to the client work), you might as well not actually be a Digitas employee. None of the professional development applies to you, HR doesn't care about your department's retention rate or team morale, and no one steps in when you're drowning and need help to do your job efficiently.