based on these responses, I gather you both had a positive experience with booking agents.
Paying 15% Seems like a no-brainer to me. 85% of something is better than 100% of nothing any day.

The only way it wouldn't make sense is if they signed you to an exclusive agreement then didn't make any effort to book you, but demanded 15% of gigs you booked yourself.

It has always seemed to me that most musicians are not good businessmen. Those who are do OK, but the rest probably ought to have an agent.