Pros
There are plenty of fun, vibrant agencies in London, but atom offers something different - a family, combined with an extremely effective team. Andy's vision when he started Atom was to create an agency with minimal politics but maximum feedback, where people can express how they really felt and use that to make things better, where we "start with the end in mind" to make sure we're doing work which really makes sense/makes a difference, and where we genuinely care about each other and our environment. It's clear to see from the comments below that he has achieved this goal, and created an amazing place to work. Working at atom provides a huge amount of opportunities to work on well known clients, learn a broad range of digital marketing skills, develop yourself/soft skills and climb the career ladder, but by far the best thing about working at atom is the people. If something goes wrong, you're feeling down or something bad happens to you, atom will be there for you in whatever way you need.
Cons
Not a "con", but something to know - You are expected to work hard and learn fast. If you like a slower pace of life then atom may not be for you. Salaries can be at the lower end of what you see in other agencies, but there are a lot of other benefits which make up for this such as annual bonuses, up to 30 days holiday, £250 learning fund per year (to spend learning anything you like), flexible working, yoga classes, subsidised counselling, bike to work scheme, monthly company nights out/fun events, regular team lunches and nights out.