Pros
HMM is better than most firms overall. As a mid-size company, it has some elements of a big corporation including a large client base and diversity of projects, extensive in-house expertise, many offices which work collaboratively on large projects, and legitimate emphasis on company-wide professional development of each individual's skills and talents. Being part of the corporate family with Hatch and Mott MacDonald also contributes to these big-corporation pros. Positive elements that are more characteristic of smaller firm include supervisors and even high-level executives who generally listen and mentor your career, an atmosphere that can be rewarding for self-motivated workers, and good communication within groups/subdivisions such that you know everyone/everyone knows you. There are many staff who have been at the company for decades, and they've been promoted for their good work, which is promising for someone who might want to build a career here.
Cons
Some negative elements of big corporation are present at HMM, which include tons of non-billable staff and overhead expenses, which results in high multipliers for billable/wage-earning staff, who are also started off at very low salaries for the beginning of their careers. There are tons of administrative forms to fill out which take away from productive work time, and a fair amount of bureaucracy, internal competition/client hoarding, and status quo/business as usual attitudes. Some of these dynamics are exacerbated or caused by the corporate family structure, which is by no means a democracy (Mott MacDonald seems to wield the most influence). Being only a mid-size firm I think that we do miss out on some really massive, profitable, interesting jobs, especially in the practice areas of the less strong divisions/subdivisions. There's room for the company to grow on these topics. The fact that so many staff have been promoted makes the company a bit top-heavy, and there is currently an issue with cycling through entry-level staff, who have no room to move up to mid-level. This could be a self-correcting problem if there is a wave of retirement.