Pros
Klaviyo is a terrific place to work, and offers a ton of growth opportunities for its employees. In sales, London has been particularly exciting as you get to be a part of a small but growing team in a growth market for the company - you have the dual benefit of being in a smaller office where you get the chance to know your coworkers and learn from people in different roles, but you have the resources of the larger organization behind you. One of the challenges of working in tech is that you might want to be part of a small, scrappy startup, but want the security and peace of mind offered by a bigger company. Klaviyo London offers both - the team is still relatively small (under 50 as I write this), so it has the feel of a proper startup, but you aren't worried about whether or not they'll make payroll one month to the next. Truly a fun and exciting place to work. And you'll have the benefit of using their INCREDIBLE office space in the near future.
Cons
Sometimes the time difference between London and the Boston HQ can be challenging, though they've learned to manage this quite well since opening a year ago. I also wouldn't expect a simple 9-5 schedule, though don't think you're chained to your desk or anything like that - it's just a high-caliber work environment, so be prepared to bring your A Game.