Pros
GiveWell has an amazing mission, and the work it does makes a huge positive difference in the world. It's pretty cool to be a part of that! People also take the values seriously.
The people are kind, friendly, talented, and care about the mission.
There's a real commitment to work life balance - this is "night and day" different than both other nonprofits and for-profits I've worked at. People aren't working at night or on the weekends, and e.g. posting to Slack at night would be frowned upon.
Cons
GiveWell has an ambitious mission, and there's a sense of always want to grow and improve and do more. I'm energized by that vibe myself, but I think some folks might prefer a more laid back environment.