A great place for those starting out - caution advised for mid-career professionals and those outside consulting.
Pros
CH2M HILL has a sterling reputation both inside and outside the consulting/engineering fraternity, and if you're fortunate enough to be in the right position (or business group), there are opportunities to work on challenging projects with real career development potential. Many of those I've worked with who have gone on to jobs elsewhere have remarked that the CH2M HILL cachet was a key factor in landing their new position. In addition, there are many good, talented folks that work for the company. Although I did not take advantage of it, the company mentor program appears worthwhile. CH2M HILL is a great place for those just starting out in their careers, or for those who are truly committed to an engineering/consulting career.
Cons
I worked for a couple of different entities and in a couple of different offices/geographies, but the one constant and, I think, biggest drawback to working at CH2M HILL is an overall failure to assimilate folks from outside the firm. Many of the senior staff at Hill have spent a large portion, if not their entire careers, there. Mid- and senior-level folks from different backgrounds (outside of consulting) or other consulting/engineering firms aren't generally well-integrated into CH2M HILL and their perspectives and input tends to be discounted. This is a generalization to be sure, but its something I have observed and experienced. A related problem is that the thinking internally can be biased - in other words, there's a tendency for managers to be convinced of the infallibility of their beliefs/assumptions. This is particularly true when it comes to business acumen, strategic/long-range planning, personnel management, etc. Although there is a high level of technical and project management expertise in the firm generally, this unfortunately doesn't always carry over into other areas. "Soft" management skills and good people management on the part of leaders is not as common as it ought to be. Another problem I encountered is the lack of standardization between different offices/geographies/programs. There is much talk about "the CH2M HILL way" of doing things, but you'd be hard pressed to put your finger on exactly what that is. Finally, pathways to career advancement can be extremely limited. For example, during my time in Houston, I noticed a distinct preference for external hires as opposed to promotions from within.