[seanwilson]

I've used Codility and other tests before. Doing amazing on a test isn't always going to mean they're going to be amazing at the job but if they do badly on the test then they're probably not worth continuing with. Tests like these act as a good automated time saving gateway to filter out obviously unsuitable candidate as conducting good interviews takes up a lot of time. You can learn a lot from their test answers as well; chatting to a candidate about their code in the interview can be a good way to understand their coding experience.

By the way, when reviewing tests answers, I always tried to be realistic and not penalise someone too much for e.g. missing a subtle optimisation trick or slightly misunderstanding the problem.