I thought my husband's head was going to explode when this series came to PSN. He wanted to see it so badly, but the cost per episode, for a rental, was just more than we could justify. He was thrilled when the series came out on Blu-Ray.
Unlike some anime series that seem to go on forever, this series comes to conclusion after season two. You should, however, be aware that the seasons do not stand alone. Season one ends on a cliff hanger, and if you only watch season two you will be beyond confused. (Honestly, you may still be a little confused if you watch both seasons.)
The story has a lot of classic love story elements in it. There is a lot of action and a lot of conflict, but for me the center of the story is a tale that has been told over and over again. Best friends are in love with the same girl, but she's always known where her heart belongs. The side stories are as compelling as the main story; they are stories about the different ways we love each other. A mother's love for her child, an older sister's love for her younger sister, and the sacrifices we are willing to make for the ones we love the most.
If you are looking for a cute and fluffy anime, this is not for you. Xam'd is not afraid to break your heart by killing off a character they made you love, and in the end, not every character gets to live their happily ever after. There was something about this anime, although I'm not sure what, that reminded me of Eureka Seven.
Overall, I don't know if this is one of those series that you just HAVE to own, but it is well worth watching. I'm pretty sure that we never watched less than three episodes in a sitting, and we finished the second series in a weekend. It's hard to turn off, and even when it ends, you'll find yourself wanting more.