Anecdotal, but I regularly purchase apps and services that influencers post about. The influencers that come to mind for apps specifically (iOS even more specifically) are John Gruber / Daring Fireball, Loren Brichter, Louie Mantia, Marco Arment...

I think the secret to great success is knowing your own target audience really really well, so that you choose the right influencer for max ROI.