Pros
Building Blocks take on quite a lot of Junior Developers and put them straight to work on real projects. It's environment where you can learn a lot very quickly if you are talented and driven - there are a lot of helpful and smart people around to learn from - however there sometimes isn't enough support for Juniors and projects can go awry because of this. There are a lot of opportunities to work with global brands that will impress your friends! Projects are often technically interesting. City centre office location with lots of good places to go out to eat/drink nearby. Meetups are occasionally held in the office. You will get to work with a lot of great people, who are good at their jobs and fun to be around. There are regular company nights out, sometimes paid for by the company. Flexi time, overtime pay (and no expectation to work overtime), good pension scheme.
Kontras
There are often issues caused by poor specs and scope creep. Developers care a lot about the quality of their work however it is sometimes compromised by tight deadlines. There were some scheduling issues at times however this at got a lot better by the time I left. Pay is below average for the area and seems to be based more on your ability to negotiate than your contribution to the company. Some members of the management team seem out of touch with the day-to-day workings of the company.