We think it'll be useful.

You're right re the payout amount for blackhats, but it only takes one whitehat to claim a reward for a bug to get killed, and the idea of a sprint is to get a bunch of them focussed on the same target at the same time. We've been running sprints alongside the more traditional ongoing bug bounties since we started, and they're very effective.

Hopefully we will get enough individuals and companies backing this to make the rewards attractive to the right kinds of researchers.