I also liked the way it ended. I thought it was bittersweet that they had to face death, but who better to face it with than the one you love, you know?
In mythology as well as my nature-based faith, the spiral symbolizes death. Yet it's easy to forget that it's part of a cycle of death then life (if you believe in reincarnation). The entire city went back to the Earth from which they came and the cycle will begin again in umpty-hundred years, which is an interesting thought.
A lot of people like clear-cut, explainable endings. I like the fact that the mangaku left it up to the reader to feel the ending as they wish. Although this was definitely a horror manga, it seems that the ending wasn't nearly as horrific as it could have been, especially considering that whole bit with the babies and the "mushrooms". *shivers in disgust*
So that's my take on it. I liked it and thought it was somewhat sweet that the lovers die within the spirals.