Pros
Great work environment, and a genuinely creative body of knowledge. One can learn a lot at Buro Happold, and landing a job there at the beginning of one's career is great. You will develop great skills and be involved in projects that are genuinely interesting. Given that their recruitment policy is a disaster, and they under-resource every job, this also means that those with initiative can take on a lot of responsibility at a level that would be unthinkable elsewhere. There is little resistance to good ideas, regardless of where they come from, so even junior employees can contribute to the success of projects.
Cons
The management of the company is stuck in the 1970s. Staffing policy is: hire three for the job of five, make them work for five, charge the client for five, and pay them for two. Burnout is common, and opportunities for promotion nigh on non-existent. Unsurprisingly, many junior engineers leave as soon as they get chartered, taking their knowledge with them. Lucky is the company that employs them subsequently, because they will shine. At the mid-senior level, promotions are basically achieved by leaving to go to another company, as there isn't even the structure in place to foster career progression. Asking to discuss salary and career progression are actually two good ways to ensure that one is never promoted. The point about under-resourcing above remains valid here, though: engineers will often work way above their pay grade with neither the salary nor the recognition, which is a shame.