Pros
There are some perks - invites to cool events, freebies from suppliers, the brand is quite glamorous (from the outside....) which might look good on your CV for future roles.
Cons
Where to start? Speaking from someone who was at mid-management level and having seen how things operate at the top level - the company is a complete joke. The management are a laughing stock; 3 posh boys from privileged backgrounds who decided to start a company... and that's about where their expertise ends. All they care about is money so morals, producing a quality service and employing high calibre individuals went out the window a long time ago. Clients are pulling their hair out and in many territories outside London, Quintessentially has such a bad reputation that other companies refuse to work with them. Slowly but surely the cracks are forming but I don't think the founders will care as they will just sell up and sack it off to someone else. Oh and the money is dire.