First, the Gigmasters commission is 5%, not 15%.
We've been on it for about 6 years and it works well for us, but you really have to be the right kind of band I think to make GM work. Us: 6-7 piece cover wedding/corporate band offering cocktail hour and, if requested ceremony music. We get lots of inquiries, but we only take one booking per month usually. We quote generally between $1900-$2600 per wedding/corporate gig. You won't get many inquiries for clubs. GM just doesn't make economic sense for those types of venues. But for us, paying $120 GM fee for a $2400 wedding gig is worth it.
The nice thing about GM is that it gives you a built-in online press-kit. Very valuable. My sense is also they do promote their website a lot. They have thousands of entertainers. I believe they advertise in major wedding publications.
The downside as a new enrollee on GM is that it takes a while to build a reputation and therefore, prime ranking positions in your area when prospective clients input bid requests and searches. (although as a new enrollee to GM, you get put usually in 1st position, which as a GM veteran with a great GM reputation kinda stinks) Ranking is based on GM gigs booked, dollars booked and client reviews, things you don't have total control over. So, screw up a couple gigs, get a bad review and your ranking goes down.
But for a band like us choosing to play only private events, it works well. Some years, most of our bookings are GM bookings. Others, like 2013 were almost all gotten from people finding our website or from referrals or prior clients. For 2014 summer, we're fully booked for our wedding and corporate gigs, but only one of those came from Gigmasters.
It can be a good thing for the right situation.