I’m not sure it’s ever legit for the job-hunter to be paying the recruiters. It would normally be the employer.
A % commission doesn’t give that much incentive to find you the very best job as opposed to the first one that will do. You’re paying them a percentage but they’re looking at the return per hour of work they put in. You’ll come under a lot of pressure to accept the first job on offer simply because that job gives them the best return even if it is a smaller cash amount than the best job they could possibly find (if they put the time in).
Their incentives do not align well with your incentives. So best avoided, IMO.