I've seen the Droon stories around libraries since my kids started school, but for some reason none of them ever came home. They are aimed at the seven & up crowd, so I think it's because my oldest started to read fairly late, so he skipped from Ricky Ricotta to Deltora books. (For those without short children at home, that's about a four year jump from KG level to 3rd or 4rth grade.) The next child wasn't a voracious reader, so he got along with the books left lying around by his brother, and he never really developed a taste for endless series anyway. I've read at least one of Tony Abbott's standalone books (Kringle), which I liked but didn't love, so I was interested in his ginormous and wildly popular series, and when my reading-the-library quest took me to his shelf, I grabbed one of the few in-libraries books and tried it out.
Unfortunately, In the Shadow of Goll, Secrets of Droon #28 does not work as a good introduction to the series. The three main characters are only sketchily introduced, and I never managed to tell them apart for the rest of the book. It was easy to identify their special friends inside the magic portal, but then I kept failing to tell the friends apart as well. It wasn't so much that they were identical, just then they weren't interesting enough for me to remember which name went with which special powers. I got a bit confused about which armies were ones for me to root for, and the final tragic battle left me wishing I had known the young Sparr well enough to really care about his danger. I think these books would work well as reading practice for kids building up their literary muscles, but they don't have much to offer an independent adult reader. But they wouldn't be painful if the adult found herself share-reading them, or reading a few chapters out loud. So they are good for what they are, but they aren't for me.