Pros
I worked at Webcredible for over 2 years and can't fault the agency for it's approach, the autonomy you receive, the calibre of the staff, the variety and high standard of the projects you can work on, and the camaraderie of the office. If you want to immerse yourself in real cutting edge user experiences techniques, be involved in defining your role and impact, collaborate with colleagues and clients to create inspirational change and designs then this is the right agency for you. They also care about personal development and growth and make sure you are happy in your role. Plus they have unlimited holiday and a host of other benefits that you wouldn't normally get in an agency of this size.
Cons
Sometimes the internal comms from the senior management aren't forthcoming but they know that and are trying to fix it.