Pros
The benefits are great, but what's amazing is that there are truly talented people working there and you get to work with the best and grow so much. They say that 1 year at Atlassian is 3 anywhere else and I can say that without a doubt I've grown more at Atlassian, and faster, than anywhere else. And the company believes in you and lets you take chances and doesn't punish failure or bad ideas.
Cons
It has changed a lot, you can feel that the core of the company has shifted from a Sydney startup to Silicon Valley, most of the high ups are now in California and things feel more and more things enterprise. We are talking decision not based on gut and previous Atlassian experience, but rather because that's how they do things in Silicon Valley companies. Not saying that's a bad thing, it's probably important for the company which is needed. But it's a shift in culture and a focus away from the Sydney office. The pace of work can be insane, not a lot of overtime, but rather so many things going on at such a pace that it can make your head hurt.