To me, Gust games are all about a fusion of fun gameplay, cute characters, and awesome music. You have some developers like Compile Heart that tend to focus entirely on the 'cute characters' part at the expense of everything else (although they've been improving lately), but Gust usually delivers an actual entertaining game to go with any fanservice it might have.
I'm still a bit worried about Gust, really - first, the last time they strayed away from turn-based battles, we got the atrocity that was Ar tonelico 3's battle system. I'm sure they've learned their lessons, and the trailers sure look good, but it's still a bit worrying. Also, Atelier Shallie was mostly a disappointment to me. I was really disappointed at how they squandered the world setup that they had built up over the previous two games, and then basically did nothing with it in Shallie. Still, I love Gust and I want to believe that Yoru no Nai Kuni and Atelier Sophie will be great games. It's time to find out how the first one turned out now.