I like how the performance evaluation seems to be fairly realistic - I used Ruby and my algorithm could have been more efficient (O(n) space, and O(n) time but with more traversals than necessary), but I scored 100 because I met all the stated constraints. (http://codility.com/demo/results/?id=demo8XKE77-H8G if anyone wants to refer to and/or mock my code.) It seems to me "ability to solve problems well enough under time pressure" is exactly what you want a simple hands-off test like this to screen for.

In face-to-face interviews you'd probably follow up by adding constraints: "your algorithm uses O(n) space in addition to the input, can you do it in constant space?". It would be cool if Codility supported that kind of workflow - following up the submitted solution with refinements on the original question. (Relatedly - do you support the workflow where you get a second try if you didn't get 100, maybe because the evalution revealed a constraint that wasn't clearly stated in the problem outline?)