Pros
ERM is an enormous consultancy that offers stability and opportunities. You can work with high-profile clients and have a genuinely big impact in the sustainability realm. The pay tends to be better than smaller consultancies. Your experience will depend significantly on what department you're working in, as some are more balanced and better organised than others.
Cons
The decisions taken by leadership often don't reflect the sustainability values of the company. There's an inverted pyramid structure, with lots of seniors who do little and then juniors who are worked to the bone. The company seems to run on ambitious promises made by partners that were then not delivered. These same partners then fly off to the States for 'summits' and frequent oceanariums and steakhouses, whilst their teams are left to pick up the pieces of the bad decisions they've made. In my particular department, we were working on a new service that eventually failed to gain market traction, due to the cycle of compulsive overpromising and underdelivering. My team and I were lied to about our job security, and vast swathes were sent packing at a moment's notice. I was lucky to be able to jump before I was pushed.