I've been thinking about a service like this. I think it would be very valuable for "experienced" devs (think 1 year repeated n times) or devs with a few years out of school who want to join a startup or a big co and are rusty in algo's, startup engineering, cs concepts and coding interviews. Stage 2 could even work as a sort of mini dev bootcamp, where candidates could work on not only interview techniques but also on developing a portfolio, sample projects, etc. Candidates could pay for mentorship once they select a mentor to work with.