Very painless process, 2 rounds. It is very important to apply early, however. First round was screening with member of America office, very standard motivational/cv questions. Final round 5 back to back interviews at London office (they covered flight and hotel!) Some fairly basic technicals asked but mostly behavioural. WSO guide will be enough for preparation. Everyone really was nice. Heard back within a few days after both rounds. My friends who were rejected took weeks to hear back.