Great company: opportunity, people, talent and culture
Pros
I'm a mid-career engineering manager with experience at a couple of startups and several of the big tech companies (Google, Adobe, Microsoft). Atlassian has talent as good as the best of those companies (e.g. Google) and has done a better job of keeping a collaborative culture. The scale (currently ~3500) is large enough that there are a range of interesting projects and there is plenty of opportunity for career growth, but the company hasn't yet started to feel like a big company. Very little beaucracy, and no BS. On that last point, it's really been a great experience working for an Australian company -- there's an honesty and freshness that permeates the company culture that I haven't seen in any of the silicon valley companies.
Cons
For more senior folks, timezones may be a challenge -- there are several engineering teams that are split between Sydney and the Bay area.