This is a startup, and it's still very risky. The whole thing might implode next month. Probably not, but perhaps. Their customers are large companies, with the attendant slow, cash-draining sales cycles.
If they do land the sort of business they have the potential to, 2015-6 will be a series of success disasters: more and more customers, more and more to do, more and more growth, more and more change. Some of the time, the work-life balance will have to tip in favour of work.
The flip side of respecting initiative and of low-key management is that you will be left largely alone to come up to speed. People are happy to answer questions, but there is little structure to feed you training. Similarly, it's up to you to figure out where you want to take your career,
If the staff size does double in a year, then whatever the culture and the pros and cons are now, they will be different in a little while. Whatever your job is now, it will be different next year.
Salary is not lavish. But no worries, if they are successful, the stock compensation is where you'll make the real money.
The open-format office is cheap to furnish, and is in fashion. But I'll bet you'll be happy after you bring a good noise-cancelling headset to work.