Pros
The company is constantly growing at a rapid pace, both in terms of head count and customer revenue. Most people within the company are smart and bring their own specialised knowledge to create a company with great capability. The culture is great, everyone is socially laid back yet focused on performing their work to their best. Strong customer base and innovative business model that works well will fuel growth for a long time to come.
Cons
Whilst I am an itern (so this doesn't affect me), it is common perception that salaries are reduced from market rates for the luxury of working at Atlassian (which may or may be worth it depending on circumstances). With so many intelligent people within the company, you are competing against all colleagues for promotions, rather than just a handful of people if it were another company. There is a struggle to keep the culture and quality of employees during rapid growth as Atlassian has begun to saturate its reach into the Sydney tech talent pool. Some employees have the social maturity of a 15 year old, and this mindset thrives in a company without the typical corporate culture.