Why is project Euler being shown in hackernews now ? This isn't something new and has been there for long time. I often use the easier problems in project Euler for interviews to test candidates. Having said that the problems in project Euler are more math based in nature.
Can I ask how do you evaluate the candidates with these problems? I mean: do they have to solve similar problems on their daily job as well? Or what is the take-away in your specific case?
For instance I often start interview with asking candidate to write a program to find a palindrome. After they answer that my follow up question is to write a program to find the longest palindrome in a given string which is a question taken from project euler.
Here I am mainly interested in their logic and approach to problem solving rather than a workable solution first time round. I don't give take-away problems neither do candidates have to do this in their daily jobs.