I think Mary and Witch's Flower is a solid film, and I'm glad Studio Ponoc started out strong. I hope their second film will be just as good.
Some people are calling Studio Ponoc the new Ghibli, and I can see why. Like most Studio Ghibli films, Mary and the Witch's Flower is based on a British kids' novel rather then a Japanese novel, manga, light novel, or game. (Not that there's anything wrong with that.) Plus, it has a similar art style. This gives the film a similar feel to many Ghibli films. Not to mention Studio Ponoc was founded by a producer from Studio Ghibli.
I'm not sure why the creator's of this film left Ghibli to create their own studio. I do have speculations, though. It might simply be because the studio is falling do to Miyazaki's semi-retirement, and they thought it easier to start fresh. When Marnie Was There and The Secret World of Arrietty aren't as popular as other Ghibli films. Which is a shame, considering they're both pretty good.
In the meantime, I hope to see more from both Studio Ponoc and Ghibli in the future.