I've never been on an 18-30 but there is one piece of advice that I've seen people discuss before: don't blindly buy your club tickets from them.
Do your research on Spotlight about which nights are good, where you can hear the type of music you like or your favorite DJs, etc. Check the party calendar, check the clubbing reviews. Then you'll know which tickets the 18-30 guys offer that might be of interest to you, which ones to avoid, and which nights to go to on your own.
Also, read the town, village and beach descriptions on Ibiza Spotlight for some ideas about day trips you might want to take yourselves.