Pros
Copper is filled with friendly, knowledgeable people who are always willing to help. It's clear that everyone who works here is talented and puts in their best effort. If culture and being able to trust in the competency of your coworkers is important to you then look no further. In terms of size I think it's the right balance between wild west startup and overly bureaucratic behemoth. There are some processes in place, but not so much that you feel constricted in how you can do your job. You can also get to know most people in the company if you wish to.
Cons
The Toronto office is much smaller than the San Francisco headquarters, so if you work at this location you might feel disconnected from your team at times. As with any smaller organization there is sometimes a lack of organization or processes in place. This can be a pro if you're looking to impact how things are done, but can be a con if you have a greater need for structure.