Pros
Good work-life balance, capable colleagues. Quite a bit of freedom to do what you want. Good company culture towards quality of work. Good place to work if you have either no experience, or are an experienced manager.
Cons
Compensation is very low for technical staff (data science/software engineer), even when the company is doing well. The company hires a lot of PhDs, but experience gained during a PhD barely counts towards your years of experience, so you'll be starting at a similar level to someone with an MSc. Salaries are non-negotiable for technical staff. The company is very secretive about the compensation given and you are expected not to talk about it. Little motivation is given to do your job very well. You won't get compensated. Work can be uninspiring. There's also a good chance you'll be working on a client site for months on ends with little interaction with Tessella. Not much room for growth (they make is seem like there is, but in reality if you want to enter middle-management you need to be hired externally.) Very much a 'just another number' culture, you won't be treated like an individual if you're technical staff. Constant traveling back and forth to the main office. Overall, I'd only work here if you have little experience and move on to a better paid/more inspiring job after. However, if you have a lot of experience and can enter into a management/high-level consultancy position it may be a better experience.