Pros
Financial rewards are very good indeed when compared with other software companies in London and the rest of the UK, particularly the starting salary. Other benefits are also great, there is a good pension scheme and free lunch, snacks, fruit and drinks are provided every day. There are lots of opportunities for growth in both technical knowledge and career (such as management roles) for those who perform well (I was able to achieve a senior role and management position within a few years thanks to high performance). Employees are encouraged to take on projects outside of their immediate responsibilities in order to help them grow, which also brings some interesting variation to the job. The environment is definitely engineering focussed and very relaxed, senior managers chat and have lunch with new recruits. There are regular social events that are fully paid for by the company, things like go-karting, bowling, etc, which are great fun and many people regularly socialise together after work too. This kind of environment has been very hard to find in software enginering jobs in London in my past experience.
Cons
Pay is not as good as the banks (although it is close), this is a software company, if you want to work all hours of the day and be rewarded with a huge pay cheque then go to a bank. FactSet makes up for this by being focussed on engineering and encouraging a good work / life balance. The company can be a little slow in identifying poor performers and acting on this, which can seem unfair if you are doing very well.