Pros
As a software developer I have found that everyone is very approachable across all levels in the company. If I ever have a question that needs answering it's easy to sit down at someone's desk and get the answers I need. From a technical standpoint I value Optiver's emphasis on starting with a simple solution and incrementally building up from there when needed. Another advantage is that since our customers are our internal traders we have an immediate feedback loop where we can continually improve and tweak our applications until we have a really solid system. As far as company culture goes, I would say it is relaxed and I enjoy the optional events and activities provided outside of work.
Cons
Trading is a fast moving industry with products and strategies changing. This can put pressure on our applications to change frequently, but also provides new problems to tackle and keeps things interesting.