Thanks for posting this link! You were right, it's highly addictive
. The last couple of days I've enjoyed solving problems from this site. I found this an excellent way to train myself in number theory, as I'm somewhat weak in this area.
I liked the submission system a lot, including the fact that I don't have to worry about TLE at all (unless the programs need days to finish...) and the freedom to choose the tool I want to solve the problems.