Pros
The people are what makes ArcLight Consulting a great place to work. Everyone is incredibly smart and helpful and genuinely willing to take time our of their day to help you - and 1v1 help, not just pointing you to something to read online. People care about spreading their knowledge and helping others grow as well. Being a boutique consulting firm, you get to connect with leadership more than you may at a larger firm and sometimes get an understanding of where their mind is at for the future. They also have a pretty solid bonus structure that they modified many times to make sure it was truly the best for us and compensated us for the work we've done so that's a huge plus and something I appreciated. Also, if you want to switch specialties or just learn something different, they let you jump on that almost right away so there's a lot of flexibility to make it what you want. If you're looking for a boutique consulting firm experience without big company politics, a place to make a defined impact as well as some pretty great friendships, ArcLight is a great place to be.
Cons
Small firm can have long hours and you can feel like you're doing more than you should in your role at times. Thats just the nature of these firms. If you're really good at what you do you'll be fine. If you're fresh out of college, you can be pretty overwhelmed by learning software, industry, navigating clients, etc. but then after the first year, its pretty much all up from there.