Pros
The company is growing quickly, and with its client base in infrastructure architecture/engineering it is likely to continue to do so. As an employee-owned company, you theoretically have more stake in the company. You can feel that you're actually working for the team, rather than just trying to make money for the shareholders.
Cons
The company is doing a lot of its growth by acquisition. This is not necessarily always the best fit, and I witnessed more than one of acquired company experience significant pain in the transition from being a small autonomous actor to a piece of HDR. I found middle management to be caught up in meetings. I know this can be a common complaint, but it is not true everywhere, and I have consistently felt (as did others around the table with me, I know) that our time was being wasted. I found it insulting to be on a monthly internal teleconference with hundreds of people, having our time wasted by hearing who had reached their 6-month anniversary with the company, etc. I occasionally find the middle management (and sometimes senior management) to be astonishingly ignorant about the fields we operated in. This can be challenging when it restricts the company's organic growth because they think "there's no business in that area." This was frustrating when I knew that other firms knew there was money to be made in new areas, and were leaving HDR in the dust. I think this speaks to an overall culture of conservatism coming from the headquarters. The resistance to change at HDR may cause problems for the company in the long term. I think the culture at HDR is also one that can easily get stuck on the details at the expense of the business as a whole. I sometimes felt that so much time was being spent attending to the details of the extent to which someone was "making the numbers" that it interfered with the ability of people to actually do their jobs, to sell and execute the work. Making the numbers is obviously important, but I sometimes got the sense that rather than doing good work and letting the numbers follow, there was pressure to make the numbers, and doing good work was secondary.