At least speaking from observing the market and the available dApps, there is close to 0 usage right now. The main reason is, in my opinion, that smart contract fees are higher than almost any possible computation in the current categories. Meaning that the marketplace would never be used by anyone with economic requirements as long as the cost of transaction > cost of computation. I know that there is the possibility of calling iexec functions from smart contracts which needs to run on-chain anyway, but I haven't seen any example of these and can't think of any near term applications for this.
I'd be happy, if you have examples which disprove my point, but realistically I think the main point of iExec is enabling cheaper computations for massively parallel tasks, OpenMOLE would be a good example.