It's tempting to label Simoun as "Last Exile with lesbian priestesses," but that's doing the show a huge disservice. There are flying ships, yes, as well as an impressively realized world that forms the backdrop for a war between nations. But that would assume that the show is your typical high-flying adventure story, and Simoun is anything but that.
What it is: a character-driven drama. An examination of religious purity and war, and how the two entwine. An occasionally nightmarish war story, in which children are pushed to the breaking point. The downfall of a nation and its culture, and the handful of not-quite-adults caught up in it. A story about adolescence, the dangers of growing up and the necessity, at times, of letting go. Finally, a romance, although you might not realize exactly how much you've come to care until its finished. If, of course, it ever really finishes.
There are instances where it is clear the show did not have much of a budget—it was produced by Studio DEEN, after all, who aren't known for great visuals. But Simoun is remarkably daring and multifaceted never the less, and takes chances that few other productions even attempt. Its creators very clearly loved what they were doing, and their love comes across in every way from the beautiful character designs, the attention to detail, the remarkable music and even the Simoun themselves. Some viewers might feel the show moves too slowly, or that the characters spend too much time mediating on how they feel than on taking action, but really, if you came to Simoun for battle scenes then you're watching the wrong show. The show has its imperfections, but they don't take away that much from the whole.
It would be tempting to call it a remarkably good yuri anime, but it's better than that. It might be even more tempting to label it as a guilty pleasure—how could a silly show about flying lesbian priestesses be anything more than a fanservice-laden romp?—but it's better than that. You could call it a cult classic, but I'd argue it deserves better than that too. Simoun is a classic of the medium, and if you're looking for something a bit different I'd recommend wholeheartedly that you check it out. There really isn't anything else quite like it.