A number of start-ups have either already failed with a very similar business model or failed to go exponential growth. There was a good "Lessons Learned" post by one of them recently here on hacker news that got comments by both hiring managers and hackers. Why the rehash, again? What is this start-up really doing differently?
I personally wouldn't be motivated to use it either to find work or to hire. I think, when hiring, you really need to do a face to face interview. I've found resumes work well enough for pre-screening that it's not a pain point for me. And, it's not like there are so many candidates out there looking for work that it's impractical to do face to face, or at least telephone/skype, interviews with all the good candidates. I learned the lesson that face to face hiring is needed by hiring someone based on a co-worker's recommendation (even though he did badly in an interview, I looked passed it based on the recommendation). It's so costly and painful to make a hiring mistake that there's no way I would deviate from the current model and try a new method or service.
> it's not like there are so many candidates out there looking for work that it's impractical to do face to face
That really depends on your target market. For the most popular technologies (.Net, Java etc) in the most popular areas (London, Glasgow etc) for low-to-medium positions, you'll get tons of decent candidates.