Pros
Velir routinely takes on big, complex web projects, and is incredibly thoughtful in fleshing out functional requirements, and does very mature development work. Awesome group of people there — both leadership and staff — it's not easy to get hired at Velir, but that's a good thing. They tend to only hire really smart people, and it's good to be surrounded by others that you can learn from. The amount I learned from the time I started was immense — not sure I've ever grown professionally as much as when I was at Velir.
Cons
Work / life balance can at times be tough, but this is typical for web agencies — management, though, likes to do cool stuff for employees who end up having to work overtime (and the CEO is a Cirque du Soleil nut, and those outings are a blast). Largely a .NET shop — not a con for everyone, but not my cup of tea.